Analyzing Fly Ball Metrics: Pitchers Who Could Bust in 2021

Welcome to the 2021 offseason! This past season was different and exciting, offering a new flavor of baseball with definite impacts on fantasy baseball. The shortened season brought forth some new strategies from a fantasy perspective and smaller sample sizes from an analytics perspective. That being said, assuming a "regular" 2021 season, it makes sense to approach analyzing potential risers and fallers much as we have in the past.

An increasingly important phenomenon to consider for pitchers over the past several seasons has been the Fly Ball Revolution, or the proclivity for hitters to actively try to hit the ball in the air in order to beat shifts and get higher-impact hits. In this case, what benefits the hitter hurts the pitcher, so analyzing pitchers' batted-ball profiles has become increasingly more important for fantasy purposes.

For this article, I will take a deep dive into a few pitchers' batted-ball profiles from 2019 to 2020, specifically looking at stats around fly balls (home run-to-fly-ball ratio, fly-ball rate, average launch angles, etc.) to identify pitchers who could see some negative regression/decreased performance in the 2021 season. To be considered for this article, a pitcher needed to have at least 50 batted-ball events in both 2019 and 2020 (per Statcast).

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With the introduction and explanation out of the way, let's take a look at several pitchers who could see a decreased performance in the 2021 season.

Tyler Mahle (SP, CIN)

Tyler Mahle was pretty much an afterthought for the 2019 season but was a surprising fantasy contributor for the 2020 season. The 26-year-old went 2-2 over nine starts with a 3.59 ERA, 1.15 WHIP, and 29.9% strikeout rate. Several changes in Mahle's batted-ball profile stood out from the 2019 to 2020 seasons, particularly in his FB%, HR/FB rate, and average launch angle. What about these changes stand out as warning signs?

First, Mahle saw a dramatic increase in his FB % from 2019 to 2020. He allowed fly balls 22.7% of the time in 2019 but 32.8% in 2020, a whopping increase of 10.1 percentage points. This jump can likely be attributed to Mahle's average launch angle, which ramped up from 9.1 degrees in 2019 to 20.8 degrees in 2020. As mentioned earlier, contact has the potential to be more damaging when it results in balls hit off the ground, so this is not necessarily a good sign. The jump in fly balls makes sense when looking at Mahle's pitch location, as he tended to throw his fastball (54.9% usage) in the top of the strike zone.

In contrast, Mahle's HR/FB rate was essentially halved from 2019 (20.8%) to 2020 (10.3%). In other words, he allowed fly balls at a much higher rate in 2020 but had a much lower percentage of those fly balls leave the park. These seemingly counter-intuitive stats could make sense if Mahle decreased the velocity of contact he gave up. While Mahle's average exit velocity did decrease from 2019 (89.3 MPH) to 2020 (87.4 MPH), it did not decrease enough to explain the high FB rate and low HR/FB rate. Mahle's 4.07 SIERA, which takes into account batted-ball profile, supports this assessment.

All in all, Mahle seems to have outperformed his batted-ball profile in 2020. If he continues to throw the ball in the top of the zone, he should continue to allow a high FB rate. Unless he can allow softer contact, it seems likely that Mahle will allow damaging contact, which would hurt his performance over the course of the season.

Zack Wheeler (SP, PHI)

Zack Wheeler has been a useful fantasy pitcher throughout his career, but 2020 was a career-best. Wheeler made 11 starts, posting an impeccable 2.92 ERA with a 1.17 WHIP and an 18.4% strikeout rate. Perhaps the biggest change in Wheeler's profile was his average launch angle, which dipped from 12.3 degrees in 2019 to a mere 3.8 degrees in 2020. Is Wheeler's 2020 success sustainable?

Based on which article he is included in, you can probably guess that the answer to that question appears to be no. This is not to say that Wheeler will not be a fantasy contributor in 2021, but that he may not be as superb as he was this season. The theme for Wheeler here is that most of his stats simply exceeded his career marks by too much to consider them sustainable. With regards to his batted-ball profile, the stats that stand out are his FB rate and his launch angle.

Wheeler's career average launch angle since Statcast began tracking the stat in 2017 is 10.2 degrees; his mark in 2020 was 3.8 degrees. As such, it makes sense that his 10.8% FB rate was much lower than his career 20.6% mark. The interesting thing here is that there doesn't seem to be anything to explain the launch angle drop. His main pitches (fastball and sinker) had similar velocity, spin rates, and pitch location compared to 2019. Further, despite the decreased launch angle, Wheeler's 4.07 SIERA suggested that he outperformed his underlying metrics. His 2020 SIERA was one of the only stats that aligned with his career mark of 4.04.

Everything about Wheeler's 2020 season suggested a best-case scenario, making it unlikely that it will repeat itself. Given the lack of changes in his game, I would expect to see negative regression to his career numbers in 2021. While not the focus of this article, one stat that did drop for Wheeler was his strikeout rate (career-low of 18.4%), which only adds to the concern of decreased performance next season.

Justus Sheffield (SP, SEA)

This pitcher has been on fantasy players' radars for some time now and put forth a solid showing in 2020. Justus Sheffield made 10 starts, going 4-3 with a 3.58 ERA, a 1.30 WHIP, and a 20.7% strikeout rate. His fly ball metrics also looked fine on the surface, as he had an average launch angle of 9.6 degrees and a FB rate of 16.1%. If everything looked ok for the 24-year-old, why is he being included in this article?

Sheffield's inclusion in the potential fallers article wasn't because of changes in his launch angle or fly balls allowed, but because of the lack of damage he encountered from those fly balls given how much hard contact he allowed.  Sheffield's average exit velocity and hard-hit rate were in the bottom 12 and eight percent of baseball in 2020 and were similar to his 2019 marks. His launch angle and FB rate were also similar to his 2019 values.

However, his 0.33 HR/9, 4.4% HR/FB rate, and 3.58 ERA were all much lower than what he put forth last season (1.25 HR/9, 15.6% HR/FB rate, 5.50 ERA). All the evidence strongly suggests that Sheffield outperformed his batted-ball profile in 2020, and his 4.55 SIERA supports this sentiment. If Sheffield has a similar batted-ball profile in 2021 to what he had the previous two seasons, there is no reason to think that he will not see negative regression.

Walker Buehler (SP, LAD)

I've saved the best for last, as this was the trickiest/most interesting case for making this article. On the surface, Walker Buehler had another great season, posting a 3.44 ERA with a 0.95 WHIP and a 28.6% strikeout rate over eight regular-season starts. Not to mention he pitched out of his mind in the playoffs, helping the Dodgers to a World Series championship. Buehler is obviously a higher-end fantasy pitcher, but there were some signs in his batted-ball profile that suggested he may have gotten a bit lucky. It is imperative that fantasy players get proper value out of their early-round picks; is Buehler a guy who could fail to deliver?

Buehler saw his average launch angle go from 12.5 degrees in 2019 to 19.5 degrees in 2020, which corresponded to an increase in FB rate from 25.1% to 29%. Again, this is not necessarily a bad sign when considered in isolation; fly balls combined with soft contact are not as worrisome and Buehler does pitch his home games in a pitcher-friendly park. However, he gave up rather hard contact in 2020. His 90-MPH average exit velocity was in the bottom-20 percent of baseball. Given how hard he throws, it is not that surprising to see how hard the ball came off the bat when hitters did make contact.

Buehler's launch angle and exit velocity led to him surrendering a lot of home runs overall (1.72 HR/9) as well as in relation to the proportion of fly balls he allowed (17.9% HR/FB rate). These were both big jumps from his 2019 marks (0.99 HR/9, 12.2% HR/FB rate), bringing concern. Buehler's season-long numbers were also salvaged by a strong September. His first five starts resulted in a 4.32 ERA with a 1.80 HR/9 rate, and while his 1.54 ERA in September helped overall, his 1.54 HR/9 rate in September still left something to be desired.

Overall, Buehler did a lot of things well once again this season, but had enough hitches to question whether or not he can return early second-round pick value in 2021. If he continues to allow hard contact in the air, his fantasy ceiling could be limited despite his strong strikeout metrics.

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