Fantasy Baseball Starting Pitcher Arsenal Changes (Week 21): Pitch Mix, Velocity, and Movement

Welcome back, RotoBallers, to my weekly "Starting Pitcher Arsenal Changes" article series for Week 21 of the 2024 fantasy baseball season. Today's article will discuss David Peterson and Grant Holmes. For those new to this column, each week we cover a few starting pitchers and their arsenal changes via pitch mix, velocity, and movement. We're at a point where the pitch movement, arsenal, and velocity should tell us a story with several starts under their belts. With more injuries likely coming soon, we'll want to continue identifying streaming pitcher skills or struggling pitchers with underlying metrics suggesting better results coming soon.

We have a little over a month left in the fantasy baseball season, causing fantasy managers to make crucial decisions for the final stretch. Pitchers can make notable changes in smaller samples, so let's see if we can find something with these two starting pitchers finding success and understand why. Let's look through my process of examining starting pitchers and their arsenals.

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David Peterson Bumps Up Four-Seam Usage in August

David Peterson used his four-seam 27.2 percent of the time in 2023 and 26 percent in 2024. In August, Peterson bumped up his four-seam usage to 36.5 percent. He traded fewer sinkers (17.6 percent) for more four-seamers and changeups (21.8 percent) in August compared to July.

It could be matchup-related or fluky, but Peterson's four-seam movement profile shifted slightly in August. Peterson's four-seam lost about an inch of induced vertical break (IVB) at 12.1 inches compared to over 13.3 inches in the previous three months he pitched in 2024. Since his four-seamer lost IVB, the heater added downward movement (over an inch) and gained arm-side movement (3-5 inches).

It's worth noting that Peterson's four-seam lacks average IVB, and we may want to pay more attention to the four-seam adding horizontal movement. Theoretically, Peterson's four-seam would elicit weaker contact with more arm-side movement, though not as much as the sinker, which can be solid. In August, Peterson's four-seamer allowed a .396 wOBA (.465 xwOBA). That's the worst monthly results since May 2024.

That might indicate Peterson shouldn't throw more four-seamers since hitters have been crushing it. That's especially true when he throws it in the upper third of the zone, with a .514 wOBA (.523 xwOBA) in August 2024 compared to a .275 wOBA (.303 xwOBA) in July and throughout the 2024 season (.352 wOBA, .408 xwOBA). Since Peterson's four-seam has below-average IVB, he probably shouldn't increase the heater usage, especially thrown in the upper third of the zone.

What's Going on With Peterson's Slider?

Peterson's slider led his arsenal from a swinging strike rate standpoint, evidenced by his 17.8 percent swinging strike rate in 2024. That's over three percentage points below his slider's SwK career average of 21.3 percent. The main issue with Peterson's slider seems to be how often he throws it in the zone, with a 46.7 percent zone rate in 2024 compared to 40.6 percent in his career.

Peterson's slider struggles in the zone, with a .400 wOBA in 2024 and .443 wOBA in 2023. When he throws the slider outside the zone, it allows a .271 wOBA (.187 xwOBA) in 2024 compared to 2023 (.245 wOBA, .236 xwOBA). Since the slider's movement profile hasn't changed much, it's likely location-based.

In 2024, Peterson locates his slider 38.8 percent of the time in Zone 13, down and inside the right-handed hitters, resulting in a .328 wOBA. That's nearly a 10 percentage point difference from 2023, when he threw the slider in Zone 13 over 48 percent of the time, leading to a .180 wOBA. Since the release points haven't changed, we would expect Peterson's slider to bounce back from a swinging-strike perspective if he fixes the location issues.

More Downward Movement on Peterson's Changeup

Peterson's changeup added nearly three inches of downward movement while losing an inch of arm-side run. It's the second-best pitch in his arsenal from a swinging strike rate standpoint at 14.3 percent, similar to 2023 (14.5 percent). The changeup results improved in 2024 versus right-handed hitters, with a .227 wOBA (.337 xwOBA) compared to 2023 (.419 wOBA, .369 xwOBA). However, the expected wOBA suggests luck factors in his favor.

That's evident in Peterson's changeup luck factors, allowing a .094 BABIP and 12.5 percent HR/F in 2024, significantly better than in 2023 (.361 wOBA, 21.1 percent HR/F). Meanwhile, Peterson tweaked the changeup location in 2024 by lowering the zone rate to 36.5 percent, over 10 points below his 2023 zone rate (47 percent).

Peterson raised his vertical release point by around one inch compared to 2023 and 3-4 inches higher than in 2021 and 2022. We've seen a similar trend with a 2-3 inch change in his horizontal release point. That would explain the added downward movement on the changeup, giving a slightly higher over-the-top release point while maintaining the three-quarters arm slot.

Summary

Peterson's control improved, with a 36.1 percent ball rate in 2024 compared to a career 38.5 percent ball rate. Unfortunately, his WHIP is awful with better control, meaning he probably gives up too many hits. Peterson's xERA is around one run worse than his actual, suggesting he's been fortunate with a high strand rate. Peterson's overall SwK dipped to 10.9 percent from 13-14 percent in 2022 and 2023. That's partly due to him losing slider whiffs.

If Peterson can adjust the slider locations to be more down and below the zone like in the past, we might witness a bounce back in whiffs. The changeup release points contributing to having more downward movement would be a bonus if it led to more whiffs. Peterson's results have drawn us in, but the arsenal and skill factors make him a tepid recommendation.

Grant "Breaking Ball" Holmes

Grant Holmes has been a minor league journeyman, with four seasons at Triple-A since 2021. In Holmes' recent start against the Rockies, his slider elicited a 31 percent swinging strike rate, with the curveball at 40.9 percent. That's around 10 percentage points or more than his season-long averages, evidenced by the slider's SwK of 20.3 percent and curveball at 21.9 percent.

Though the graph above shows the rolling whiff rates by pitch, it still paints a similar picture, with the two breaking pitches being dominant offerings. Against right-handed hitters, Holmes throws sliders 30.8 percent of the time, translating to a .241 wOBA. His curveball is his third-most-used pitch against righties, resulting in one of his best versus same-handed hitters, with a .189 wOBA.

Meanwhile, he bumps up the curveballs versus lefties, throwing it 31.1 percent of the time. Holmes' slider is his third-most thrown one at 13.9 percent, tied with the cutter (13.9 percent). Unfortunately, the results haven't been great against the breaking pitches versus left-handed hitters. That's evident in a .381 wOBA via the curveball and a whopping .358 wOBA against the slider versus lefties.

The curveball elicits a 23 percent SwK to lefties, with a 21.2 percent SwK against the slider. Though Holmes' breaking balls still elicit a decent chunk of whiffs to either side of the plate, we often find pitchers who throw tons of breaking balls struggling against opposite-handed hitters. It's worth noting that we're dealing with small samples for the rookie starting pitcher.

Holmes' Interesting Four-Seam

Holmes' four-seam performed well against lefties (.144 wOBA, .291 xwOBA) and righties (.179 wOBA, .204 xwOBA), but the quality of contact versus left-handed hitters suggests some regression. It's his third-best pitch for whiffs, with the four-seamer eliciting an 11.3 percent swinging strike rate. We might expect it to have an above-average movement profile when a pitch performs well. However, that's not the case with his four-seam, evidenced by the below-average IVB of 14.3 inches with 6.4 inches of arm-side run.

Since Holmes' four-seam doesn't possess the movement profile that we typically find, there's a chance the results regress. Even the vertical release point and extension don't suggest Holmes "hides" the four-seamer, making it challenging to square up. Furthermore, it's logical for him to use the breaking balls often, with the below-average four-seamer based on the shape and movement profile.

Summary

Holmes has an xERA (3.44) slightly better than his actual, suggesting the skills look legitimate, with a ridiculous 17.2 percent swinging strike rate. He rocks an elite ball rate of 31.3 percent, suggesting the best of both worlds involving high-end control and strikeout skills. It begs the question of why Holmes hasn't received a chance to pitch in the majors after spending 10 seasons at various levels of the minors with three organizations. Spencer Schwellenbach caught my attention based on a deeper look at his arsenal, and Holmes falls into that bucket for the last month and change of the season. Holmes deserves more attention as a streamer with juicy strikeout skills.

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