Since the historic 19-strikeout no-hitter he threw in 2019 as a member of the Vanderbilt Commodores, Kumar Rocker has been one of the most well-known pitching prospects in all of baseball. Drafted third overall in the 2022 MLB Draft by the Texas Rangers, Rocker was expected to be an integral part of the Texas Rangers‘ starting rotation for the foreseeable future alongside fellow Vanderbilt teammate Jack Leiter. While preseason projections were optimistic regarding Rocker’s impact on the starting rotation this season, with OOPSY projecting Rocker for a 3.45 ERA and 18.8% K-BB, Rocker, similar to the Rangers offense, has underperformed these expectations so far this season, currently producing a 5.66 ERA and 12.2% K-BB over 12 starts and 55.2 innings pitched so far this season.
This article will take a look at Rocker’s pitch arsenal, evaluate why he has underperformed his preseason projections so far this season, and seek to identify if there are any adjustments he can make to improve his arsenal over the final two months of the season.
Statistics as of the end of play on July 23, 2025.
Rocker made his major league debut during the final month of the 2024 season, starting three games and producing a 25.5% strikeout rate, 10.9% walk rate, and 14.5% K-BB over 11.2 innings pitched. Rocker’s ability to generate both swing-and-miss and sub-optimal contact, as shown by his 51.5% ground ball rate, resulted in a 3.86 ERA, providing optimism that he could build upon the success of his debut with a breakout season in 2025, and promient projection systems (such as Steamer, THE BAT, and OOPSY) all projected Rocker to produce a sub-4.00 ERA, with a >16% K-BB this season. This breakout season has not taken place, however, as Rocker is currently producing a 19.5% strikeout rate, 7.3% walk rate, and 12.2% K-BB over his first 55.2 innings pitched of the 2025 season, resulting in a 5.66 ERA.
The movement plots above depict Rocker’s arsenal over the past two seasons. Last season, Rocker’s arsenal consisted primarily of three pitches: a four-seamer, a slider, and a sinker, while this season his arsenal has expanded to include six pitches: a sinker, a cutter, a four-seamer, a slider, a curveball, and a changeup. The most notable pitch in his arsenal since making his major league debut has been his slider, which possesses a “downer” or “deathball” shape. These types of sliders can be particularly effective at generating swing-and-miss because their movement profile of having more drop than a typical bullet slider causes the pitch to approach the plate at a steeper vertical approach angle than other slider variants, causing opposing hitters to frequently swing over these offerings. Last season, Rocker’s slider displayed 3.8 inches of vertical drop with 0.7 inches of arm-side run at 84.1 mph, generating a 50.0% whiff rate at the major league level. This season, the offering has displayed slightly less drop and slightly more arm-side movement than last season, generating a 40.5% whiff rate in 2025.
Rocker has also altered the usage of his fastballs this season, lowering the usage of his four-seamer from 43% to 18%, and increasing the usage of his sinker from 17% to 27%. Rocker’s sinker has consistently graded higher than his four-seamer (which possesses “dead zone” shape) in pitch quality models, and this revamped fastball usage, alongside the addition of the cutter to his arsenal, has allowed Rocker to utilize multiple fastballs to both right and left-handed hitters so far this season.
Pitch quality models, which evaluate pitches solely on their velocity, spin, and movement characteristics, are generally mixed on whether these changes have been positive or negative for Rocker’s arsenal. FanGraphs’ Stuff+ and my pitch quality model, aStuff+, evaluate Rocker’s arsenal as having largely the same grade from 2024 to 2025, while StuffBot (which evaluates pitches on a 20-80 scale) believes that the quality of Rocker’s “stuff” has declined from a 52 in 2024 to a 43 in 2025. Each model favorably grades the sinker while evaluating the remainder of the pitches in Rocker’s arsenal as around league-average. Notably, FanGraphs’ Stuff+, which takes into account fastball usage, grades Rocker’s new cutter as the best pitch in his arsenal at 104 Stuff+.
While on the surface it may appear that Rocker has been closely following two of the latest trends in modern pitching development, expanding your arsenal and throwing multiple fastballs, Rocker has seemingly made these adjustments at the expense of removing his best pitch, his downer slider, from his arsenal. The table above depicts Rocker’s usage rates before his stint on the injured list at the end of April, and his arsenal upon return from injury on June 4. At the beginning of the season, Rocker was utilizing his slider 38.1% of the time, while not utilizing the cutter despite utilizing the offering in spring training. Since returning from injury in June, Rocker has essentially replaced his slider with his cutter, utilized the cutter 40.1% of the time, and lowered the slider usage to just 2.4%.
Why did Rocker decide to replace his slider with a cutter? One reason could be to improve his performance against left-handed hitters. Over his first five starts of the season in March and April, Rocker produced a 10.4% strikeout rate, 6.3% walk rate, and 5.77 FIP against left-handed hitters, compared to a 23.4% strikeout rate, 4.3% walk rate, and 3.09 FIP against right-handed hitters. The wide discrepancy in platoon splits can likely be attributed to the fact that left-handed hitters were making a lot of contact against Rocker’s fastballs, with his four-seamer generating a 4.0% whiff rate and his sinker generating a 6.1% whiff rate against left-handed hitters in his first five starts of the season. By adding the cutter to his arsenal, Rocker now possess a fastball that is ideal to utilize against left-handed hitters, as the glove-side break of a cutter moves into a left-handed hitter, which can cause the hitter to get “jammed” resulting in sub-optimal contact, and the presence of three fastballs in his arsenal forces opposing hitters to decipher between which fastball Rocker is utilizing at any given time. Since returning from injury on June 4, Rocker’s four-seamer is generating a 25% whiff rate, his sinker is generating a 16% whiff rate, and his cutter is generating a 39.4% whiff rate against left-handed hitters.
Another reason why Rocker decided to replace his slider with a cutter could be attributed to his superior ability to locate his cutter in ideal locations. The heatmaps above depict the locations of each of the pitch types in Rocker’s arsenal so far this season. While Rocker’s slider has frequently leaked over the heart of the plate so far this season, Rocker has been able to consistently locate his cutter down-and-away to right-handed hitters and down-and-in to left-handed hitters, ideal locations for generating chases out of the zone. These heat map observations are backed up by the underlying data, as while both the slider and cutter have generated similar zone rates this season (45.3% for the slider, 46.1% for the cutter), the cutter has generated more chases, with a 42.4% chase rate, compared to the slider’s 30.5% chase rate.
While looking at the heatmaps, it appears that Rocker has room to optimize his locations on other pitches in his arsenal as well, as his four-seam fastball and sinker have a tendency to be located in the heart of the plate. This is especially an issue for the four-seam fastball, as given the pitch’s well below-average shape, it is particularly vulnerable to allowing damage when located in the heart of the zone. Commanding a baseball is very difficult, with the average miss distance of a pitch thrown at the major league level being ~12 inches; however, improving the locations of these offerings could help Rocker raise his overall level of production moving forward. Locating his four-seamer up in the zone could allow the pitch to generate more swing-and-miss, and locating the sinker more inside to right-handed hitters could allow him to generate more sub-optimal contact, as the pitch will be approaching the hitter at a sharper angle.
At first, I thought that Rocker was making a mistake by replacing his slider with a cutter, but upon further examination, this seems to have been a necessary adjustment that he needed to make against left-handed hitters. Given the fact that his two existing fastballs were sub-optimal offerings to use to left-handed hitters (the four-seamer because of the “dead zone” shape, the sinker because of platoon splits), the cutter allows him to possess a fastball that can effectively be utilized to hitters of the opposite handedness, and the shape is versatile enough to also be used as a swing-and-miss weapon against right-handed hitters. I am curious to see if Rocker reintroduces the slider into his arsenal, without ditching the cutter, over the final two months of the season. Perhaps Rocker simply isn’t able to throw both pitches at the same time in his arsenal without the shapes blending into each other, however, I believe that the additional challenge hitters would face between deciphering the slider and cutter would allow for both pitches to “play up” due to tunneling effects, likely allowing Rocker to generate even more swing-and-miss moving forward.
OOPSY is optimistic about Rocker’s rest-of-season performance, projecting him for a 23.3% strikeout rate, 7.5% walk rate, 3.90 ERA, and 3.81 FIP over the final two months of the season. “Traditional” regression indicators also indicate that Rocker is due for some positive regression over the remainder of the season, as his 66.3% left-on-base percentage is below league-average, and his high home run to fly ball ratio (16.1% HR/FB overall, 25% HR/FB against right-handed hitters since June 4) should regress closer to 10% over the final two months of the season. If Rocker can maintain command of his new cutter while re-integrating his slider into his arsenal, there is a realistic path for him to improve his overall level of production over the remainder of the season.
While the results haven’t quite been there so far in 2025, the underlying data suggest that Kumar Rocker has made some effective adjustments to his pitch arsenal, which should allow him to more effectively face right and left-handed hitters, potentially solidifying his role in the Texas Rangers starting rotation moving forward.
