Starting Pitcher K-BB% Risers: Who to Believe In

The 2020 MLB season may have left starting pitchers with shorter starts and fewer total innings than usual, but arsenal adjustments and pitch improvements were still prevalent. As a result (and sometimes as a result of other factors), several pitchers saw significant changes in their strikeout and walk rates.

K-BB% is one way to measure pitcher improvements (or declines) by focusing on factors that pitchers tend to control. Although technically similar to K/BB, K-BB% does a better job of acknowledging the relative importance of strikeouts and is more predictive than K/BB, attributes that make it a solid proxy for pitcher performance.

Below are the top 10 K-BB% risers from 2019 to 2020 among starting pitchers with at least 40 innings pitched in each season. The analysis of those pitchers has been organized based on their likelihood of maintaining their improvements into the 2021 season.

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Top K-BB% Risers Between 2019 and 2020

Each of the pitchers listed in the table above has their improvements broken down in one of two sections below. The first section is comprised of pitchers who are more likely to see their K-BB% improvements stick into 2021, while pitchers listed in the second group are less likely to see their K-BB% improvements stick into 2021.

There’s a reasonable amount of subjectivity involved in which section a given pitcher belongs in -- there’s no rule for the amount of kept gains required to land in one section or another -- and many pitchers may toe the line between categories. To reduce the amount of uncertainty in that regard, I’ve included a detailed breakdown for each pitcher, as well as a rough estimate for their likely 2021 K-BB%. 

More Believable Risers

Zach Plesac, Cleveland Indians

K% up 9.2 pts, BB% down 5.5 pts

Everything moved in the right direction for Plesac this season, and the 25-year-old was one of only two players to see his strikeout and walk rates improve by more than five points (min. 40 innings). Plesac has his slider to thank for his massive improvements, as the pitch blossomed into a legitimate weapon in 2020. From a results perspective, Plesac’s slider posted an impressive 43% chase rate in 2020 (29% in 2019) while maintaining a stellar 33% o-contact rate and a related seven-point increase in swinging-strike rate to 14%. The slider’s ability to force swings outside of the strike zone combined with its effectiveness allowed Plesac to increase his overall swinging-strike rate to 14.3% despite virtually no change in his zone rate and only a small change in his z-contact rate.

Under the hood, Plesac’s slider looked like a new pitch in 2020. To that end, the pitch saw its velocity jump just under 1.5 mph with a spin rate increase of more than 100 RPM (although 100 RMP isn’t that big of a jump for sliders, it is still worth pointing out). Two other factors that might also have affected Plesac’s slider are his pitch mix and a later break. Plesac has always maintained a consistent release point that allows for effective pitch tunneling, and it’s possible that he altered his pitch mix (by throwing his slider directly after fastballs more frequently, for example), making hitters less likely to pick up the pitch until it’s too late. Similarly, a later-breaking slider would mask it’s movement longer, making it more difficult for hitters to assess the pitch’s movement.

Overall, Plesac’s improvements are both well-earned and mostly sustainable. Plesac will probably see some negative regression next season -- particularly in his walk rate, which relied on his increased chase rate (driven by the slider) for its improvements -- but his K-BB% should stay up around 20% next season.

Kenta Maeda, Minnesota Twins

K% up 9.3 pts, BB% down 4.3 pts

Maeda saw his swinging-strike rate jump to a career-high 17.2% on the back of a significant pitch mix adjustment in 2020. Maeda slashed his fastball usage from 33% to 19% last season, instead featuring his changeup and slider  -- his two best pitches by swinging-strike rate by far.

Combined with the fact that Maeda’s changeup and slider are significantly more effective at getting hitters to chase outside of the strike zone than his fastball (boosting Maeda’s overall chase rate by six points), Maeda’s pitch mix adjustment makes his improved strikeout and walk rates easy to buy. Fantasy managers should expect Maeda’s K-BB% to stay around 25% next season as a result, keeping him near the top of the league.

Shane Bieber, Cleveland Indians 

K% up 10.9 pts, BB% up 2.4 points

Bieber may not have seen his walk rate go in the right direction in 2020, but the 25-year-old rode a significant increase in his strikeout rate to the best K-BB% in MLB. Like Maeda, Bieber decreased his fastball usage. And in Bieber’s case, a greater emphasis on his curveball (25% swinging-strike rate) helped propel him to a career-high 17% swinging-strike rate. 

But Bieber’s fastball also played a significant role in his improved swinging-strike rate, with the pitch’s swinging strike rate jumping from 6% in 2019 to 10% last season. Under the hood, Bieber saw his average fastball velocity increase by one mph last season and he located his fastball higher in the zone more frequently, likely contributing to his improved swinging-strike rate.

Those changes make Bieber’s elevated strikeout rate relatively likely to stick into 2021, and fantasy managers should expect Bieber to rank near the top of the league in K-BB% once again next season. That’s not to say that Bieber will maintain a strikeout rate above 40% (no other starting pitcher has ever done that in at least 50 innings), but Bieber should continue producing elite K-BB% results.

Zach Davies, San Diego Padres

K% up 7.6 pts, BB% down 0.6 pts

Davies finished the 2020 season with a fairly mediocre 15.9% K-BB%, but that more than doubled his 2019 mark and accounted for enough of an increase to push him into the top-10 K-BB% risers for the season. Davies also had one of the more significant pitch mix changes in 2020, cutting his sinker usage from 52% to 42% with effectively all of the gains going to his changeup.

That’s a huge adjustment for Davies considering just how much better his changeup is than all of his other pitches in terms of swinging-strike rate. With a swinging-strike rate that has hovered around 20% for most of his career (20% in 2020), Davies’ changeup is his only pitch to have posted a swinging-strike rate above 5% in either of the past two seasons, and his sinker tends to post a swinging-strike rate closer to 3%. It’s easy to see why Davies would see a significantly improved strikeout rate in 2020 as a result, and fantasy managers should expect much of that improvement to stick in 2021 as long as his elevated changeup usage continues.

Tony Gonsolin, Los Angeles Dodgers

K% up 3.4 pts, BB% down 5.2 pts

A five-point increase in zone rate helped Gonsolin get his walk rate down to 4%, but the same model used for Framber Valdez suggests that Gonsolin was more likely to post a 7% walk rate. Additionally, Gonsolin’s mediocre control in the minor leagues suggests that his impressive 2020 walk rate may not stick into next season. Arguably, Gonsolin’s relatively late introduction to pitching full time gives him more room to improve than other 26-year-old pitchers, but his walk rate is likely to rise at least a couple of points next season regardless.

Gonsolin’s strikeout rate increase is better supported. A five-point increase in usage rate for Gonsolin’s splitter (20% swinging-strike rate) at the expense of his curveball (6% swinging-strike rate) fueled a two-point increase in Gonsolin’s swinging-strike rate to 14%, giving him a reasonable 26% strikeout rate for the season. There’s a case to be made that Gonsolin belongs in the less believable category given all of that information, but a K-BB% near 20% is a reasonable expectation for Gonsolin next season and that’s enough to put him in this category for me.

Less Believable Risers

Framber Valdez, Houston Astros

K% up 13.5 points, BB% down 7.8 points

The only other pitcher (besides Plesac) to see his strikeout and walk rates improve by more than five points, Valdez is significantly less likely to see his 2020 gains carry over into 2021 than Plesac. Valdez became more of a sinker-baller in 2020 than he had been in the past, a change that doesn’t usually lend itself to large strikeout gains. Unsurprisingly, Valdez’s sinker posted a low 5% swinging-strike rate last season, and his 10% swinging-strike rate for the season was effectively the same as his 2019 mark and suggests a strikeout rate more in-line with Valdez’s 21% rate from 2019.

Valdez’s improved walk rate looks better, though. A quick and dirty model for a pitcher’s walk rate based on chase, contact, and zone rates yields a solid 0.74 R^2. Plugging in Valdez’s 2020 numbers to that model, the 26-year-old “earned” an 8% walk rate for the season. That’s considerably better than his 13.4% walk rate in 2019, but Valdez likely suffered from bad luck in 2019 given that the model gave Valdez a 10% walk rate based on his 2019 numbers.

As a result, Valdez’s improved walk rate will probably (mostly) stick in 2021, but his strikeout rate should fall back towards 20%. That puts Valdez’s K-BB% likely to wind up somewhere between 12% and 15% next season, slightly below the 15.5% league average and nothing worth getting particularly excited about.

Trevor Bauer, Cincinnati Reds

K% up 8.2 points, BB% down 2.9 points

I analyzed Bauer’s strikeout rate increase in a Twitter thread back in mid-September, and the takeaway holds three starts later. Namely, Bauer’s strikeout rate gains can be almost exclusively tied back to his improved fastball and adjusted fastball usage.

Neither Bauer’s called- nor swinging- strike rates changed significantly between 2019 and 2020 (and his 28% strikeout rate from 2019 doesn’t seem to be shortchanging him), but his pitch mix in two-strike counts changed drastically with his fastball usage rate in those situations up over 37% -- its highest usage rate in two-strike counts since Bauer’s first full season in 2014.

That usage increase came with a significant improvement in Bauer’s fastball performance, allowing Bauer to pick up substantially more strikeouts without a significant change in his called+swinging strike rate. Most notably, Bauer’s fastball spin rate increased more than 350 RPM between 2019 and 2020,  likely playing a significant role in its improved results. Interestingly, though, that kind of spin rate increase is something that Bauer has contended is really only possible with the use of foreign substances. Unless Bauer’s has changed since February, then it seems likely that his fastball improvements -- and much of his resulting strikeout improvements -- were the direct result of Bauer’s adoption of foreign substances.

In that case, I’d be skeptical of relying on Bauer to maintain his strikeout improvements in 2021. It’s certainly possible that Bauer came up with a different (legal) way to improve his fastball or that he’ll continue using foreign substances in 2021 without getting caught (if he used them at all in 2020), but the apparent likelihood that foreign substances played a significant role in Bauer’s 2020 strikeout gains makes him a relatively risky bet to maintain a K-BB% above 25% (or even 20%) in 2021. 

Zach Eflin, Philadelphia Phillies

K% up 10.3 pts, BB% up 0.7 points

Eflin’s entire 2020 K-BB% improvements can be attributed to an increased strikeout rate, but that came without a similarly significant increase in Eflin’s called+swinging strike rate nor any significant swinging-strike gains from any of his individual pitches. Eflin elevated his sinker usage in 2020, and although that trend should benefit his long-term fantasy value in general, it doesn’t bode well for his K-BB%.

Eflin’s sinker was his worst pitch by swinging-strike rate at 8.6%, and the pitch’s swinging-strike rate was not significantly higher than it had been in past seasons. As a result, Eflin should see his strikeout rate fall back towards 20% in 2021, taking with it most of his K-BB% gains. 

Nathan Eovaldi, Boston Red Sox

K% up 2.9 pts, BB% down 8.1 pts

Eovaldi’s gains came largely from his improved walk rate, but it seems unlikely that enough of those gains stick next season to keep Eovaldi’s K-BB% above 20%. Eovaldi has enjoyed above-average control for most of his career based on his solid 7.3% career walk rate, but that rate sat at 11.6% just a season ago and has fluctuated significantly over the past few seasons.

Granted, injuries and an inconsistent role likely affected Eovaldi’s 2019 numbers, but the upshot with Eovaldi is that even a five or six percent walk rate in 2021 doesn’t make him all that exciting of a pitcher, at least from a K-BB% perspective. There’s certainly a case to be made that that’s enough to put Eovaldi in the “believable” category, but his lackluster strikeout improvement pushes him into this category for me.

Marco Gonzales, Seattle Mariners

K% up 6.1 pts, BB% down 4 pts

Gonzales’s 2020 season compares reasonably well to his 2018 performance, at least from a plate discipline perspective. Crucially, Gonzales’s 2020 plate discipline metrics suggest that his strikeout and walk rates should be closer to his 2018 results rather than the other way around.

In particular, Gonzales’s 8.4% swinging-strike rate suggests that his 23.1% strikeout rate from 2020 is likely to drop in 2021, and his walk rate improvements are relatively poorly supported by his zone and chase rates. As a result, fantasy managers should expect Gonzales to post a K-BB% closer to 15% in 2021. 

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