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2021 Arizona Diamondbacks Draft Preview: Kumar Rocker

Might the Diamondbacks luck their way into one of the Vandy Boys at #6?

College World Series - Vanderbilt v Michigan - Game Two Photo by Justin Tafoya/NCAA Photos via Getty Images

Kumar Rocker, RHP, Vanderbilt

Height: 6’4”
Weight: 255 lbs
DOB: 11/22/1999
B/T: R/R

The Tools

FB: 60/65 SL: 65/70 CV*: 65/70 CH: 45/55 CT: 40/45 Control: 45/55 Overall: 60


Kumar is the son of Tracy Rocker, a former football player and current defensive line coach of the Philadelphia Eagles. Anyone who has glimpsed Kumar in action can immediately see he has his father’s genes. Back in 2018, Kumar Rocker was considered by many to be one of the two best prep arms in the entire draft, the other being Carter Stewart. Both carried significant signing concerns. In the case of Stewart, this did not keep Atlanta from trying to draft Stewart anyway, despite knowing ahead of time he was going to demand over-slot. In Rocker’s case, his strong commitment to Vanderbilt scared off most teams. He was eventually selected in the 38th round by the Colorado Rockies - mostly just for funsies, not unlike when Arizona used their 20th round pick in 2014 to select J.B. Bukauskas out of high school, despite him warning all of MLB he was heading to UNC in a fairly non-negotiable way. Clearly, Rocker chose to go to school and re-enter the draft in hopes of being a top-5 selection in 2021. (Stewart famously chose to go to Japan instead of the Braves, signing for more than anyone in the draft and making himself an unrestricted free agent three seasons sooner than he would be if he had signed with the Braves.)

After electing to go to Vanderbilt, Rocker hit the ground running, working from the starting rotation the day he hit campus. Rocker’s crowning achievement during his time at Vanderbilt came on June 8, 2019. Pitching in the College World Series Super Regionals against Duke, Rocker fired a no-hitter and struck out 19 Duke batters. He was a freshman. That performance propelled him to the top of prospect rankings, establishing him as the consensus 1-1 pick for the 2021 draft, a position he held until late-April or early-May of this year.

That happens to be right around the time that some prospects, such as Henry Davis, and Marcelo Mayer started making strong pushes up the board. It also happens to be when Rocker started to experience some rockiness in his performance. His control started to waver some. His changeup lost some of its depth. His slider lost some of its “muchness”, and his fastball even became a bit more hittable. This caused warning flags to go off about Rocker and how he might not be durable or that he might be injured. As time has marched on though, Rocker has continued to show he can perform and is healthy, while a number of other pitchers, including likely 1-1, Jack Leiter, experienced variations of the same problems. Now, many evaluators are starting to suspect that what is being witnessed in Rocker and other pitchers around the country is a direct result of the pandemic and the havoc it wrecked on schedules and training regimens.

On the Mound


When Rocker is locating his fastball, it can be a truly intimidating pitch, especially against right-handed batters. The velocity on his fastball sits comfortably at 95-96 mph deep into games. Rocker also possesses 99 mph reach-back. Rocker’s delivery features seven feet of extension with the added benefit of the ball exploding from his hand. Rocker’s four-seam offering features in excess of 18 inches of induced vertical break and 10 inches of horizontal run. Rocker also gets well above average spin rate of over 2300 RPM. It is a fastball that feasts at the top of the zone, especially against right-handed batters. There are those who still have some concerns about Rocker’s fastball. The first, almost universal, concern is that he sometimes loses the zone with his fastball. When that happens, all those features listed to show how explosive and appealing his fastball is, turn into nightmarish contributors of wildness. It was the loss of strike-throwing ability that allowed Duke to manage any base runners at all in that 19-strikeout no-no of his. One batter even took a pitch to the face (YIKES!). Coaching and experience will likely fix that problem, especially since it is not a persistent one. The other concern raised by some is his swinging strike rate (18%) on the fastball. When performing as a two-pitch (fastball/slider) pitcher, this is not entirely unexpected. Furthermore, Rocker is known to use the fastball to set up his other offerings and is not always looking for the fastball to blow away the hitter. This is the sort of concern that might very well be “corrected” when he begins to pitch with his third and fourth offering as a matter of course, rather than waiting until late in games to break them out.


This is where I need to stop and do a bit of explaining. For those looking far and wide for more information about Rocker, they are likely to see a number of sites touting Rocker’s dominating combination of power fastball and 70-grade, knee-buckling curve. Other sites will go on at length about Rocker’s wicked fastball/slider combination. Here’s the thing, they are talking about the same pitch. The short explanation is it is just a slider with an unusual shape. That doesn’t come close to doing it any sort of justice though. As far and away Rocker’s best pitch, it is only prudent to take the time to explain how this plus-plus pitch actually operates. The problem is, it comes down to discussing the differences between spin releases, transverse axis and gyroscopic spin, and arm angle. I was concerned I might need to devote an entire article about why Rocker’s curve-shaped slider is not a curveball. Then I found this quick and dirty explanation by Lookout Landing’s Joe E. Doyle as to why Rocker’s slider tumbles to the plate with the shape of a curveball.

This is due to gyroscopic spin. This is achieved by spinning the ball out of a pitcher’s hand like a bullet twirls as it travels. It spins on a gyroscopic axis allowing gravity to do all the work.

Basically, the pitch falls right [off] the table toward the dirt due to the nature of its spin. Conventional breaking balls spin on a transverse axis. Gyro spin is less affected by air density and resistance. The only reason this isn’t classified as a curveball (as the eye would perceive it), is because the spin axis is totally opposite. Curveballs tumble end-over-end... gyroscopic sliders spin in the same way a football does.

Thank you Mr. Doyle.

This action by Rocker’s not-a-curve slider is actually assisted by Rocker’s very average spin rate of 2250. With that level of spin, Rocker is not going to be able to get the sort of sweeping action that is commonly associated with the pitch. But, in this case, the lower spin rate, combined with the gyro-spin, allows the pitch to drop completely off the table.

As for the normal descriptive details, Rocker has a plus-plus slider that he throws at 86, though he has frequently reached 88, which is where some think he’ll be by the time he debuts. The pitch tunnels almost seamlessly off his fastball and features plenty of depth before dropping completely off the table. It has a 51% swinging strike rate and a 35% chase rate. He spots it well and will throw it to any batter in any count.


Early on, Rocker did throw what amounts to a “baby curve” that sat around 81-84 mph. He has not thrown it in some time now though and, given the excellence and shape of his slider, has probably abandoned for good. The pitch was average at best, not featuring much break on the 1 to 7 plane. This season there are reports he went back to that pitch, getting a bit more depth on the break, but I have not been able to find raw data on that, only descriptions that sound remarkably like a misidentified gyro-slider. That said, Rocker was able to throw his curve for strikes with regularity and used it well in keeping hitters off-balance. It was (and could potentially again be) a worthy average pitch.


Rocker’s change is still very much a pitch in development. Rocker has shown good intuition with how to use it and has managed to develop a feel for throwing it rather quickly. Rocker’s delivery once again gets seven feet of extension, while the change features 17 inches of horizontal run which helps him to mimic the fastball. The overall shape is quite good and Rocker is able to deaden the spin down to 1700 RPM. What’s lacking in the pitch is consistency, especially with location. Given how infrequently Rocker has been forced to turn to the pitch in competition, this is not surprising. With more reps, this should be a true plus-offering.


Giving more credence to the notion that Rocker is not planning on returning to the curve soon, he has started developing a cutter to add to his repertoire. He used it as a quality set-up pitch, allowing his already explosive fastball to play up even more. There is very little on this pitch right now, but he is showing advanced feel for the pitch and also advanced understanding of pitching as a whole, despite the fact that he could likely still be a top-10 pitcher with only his fastball/slider mix.


In 2018, Rocker’s control was a real issue, leading some to speculate that he might even wind up being better suited to being a late-inning reliever. Since then, he has done nothing but improve and put those worries to rest. He still has mental lapses that will lead to a loss of control. He can trow his slider for strikes at will, but his fastball still sometimes rides it way right out of the zone. His changeup needs more work, from side-to-side, but he is adept at keeping it low, not missing up in the hittable area of the zone. Getting consistent reps with his non fastball/slider offerings seems like the only thing he needs in order to be a consistent strike thrower.


Rocker’s advanced approach and self-awareness are the sort of assets that simply cannot be taught. He is a potential. His combination of two plus-plus pitches and potentially two more plus offerings give him a true top of the rotation ceiling. He throws from a high 34 arm angle. His wind-up features a behind the head load and a high but balanced leg kick. He drives straight toward the plate and does not fall off the mound at all. There is some jerkiness to the wind-up and it features a lot of moving parts, but he has shown a remarkable consistency with being able to repeat it, enough that some coaches may elect not to tinker with it unless it becomes a problem at higher levels.


Kumar Rocker is a big-bodied workhorse of a pitcher with the potential to be one of those, now rare, starters capable of throwing 200+ innings without showing significant fatigue. He has the ceiling of a staff ace and the reasonable floor of a big league starter with the tools to be a high leverage reliever. Rocker’s unique slider gives him a pitch that will serve him well at any level. It is a pitch with movement not seen with any regularity since Daisuke Matsuzaka left the big leagues. However, Dice-K’s “gyroball” was not nearly the hard-breaking offering that Rocker’s gyro-slider is now. Having a unique, wipe-out pitch is a rare talent and Rocker has that talent in spades. If not for the COVID-19 pandemic, Rocker would almost certainly still be the consensus 1-1 pick in the draft.

Rocker’s 19-K no-no (all the Ks)

Scouting video from shortened 2019


Rocker is a true 1-1 talent with ace potential that is the result of one of the filthiest, most unique pitches being thrown today. He bullies batters of all types and is a fierce competitor. His raw athleticism is mostly unmatched in this draft, though Brady House says hello. It is difficult to imagine any scenario in which Rocker falls lower than Arizona at #6

Chances Rocker reaches #6: ~40%
If Rocker is still on the board, chances Arizona selects him: 95%+

Up next: Henry Davis, C, Louisville