The Red Sox Fixed Kyle Harrison's Arsenal. Is He Primed to Soar for the Brewers?

While he was with the San Francisco Giants, Kyle Harrison was predominantly a four-seam fastball and "slurve" pitcher, adding the occasional changeup to right-handed hitters. For large parts of his development through the minor leagues, Harrison overpowered hitters with his funky low-slot delivery and the deception of that heater-slurve combo. In the low minors, he missed lots of bats, and he peaked as the No. 18 prospect in baseball, according to MLB Pipeline in 2023. Here's the blurb that went with that lofty ranking.

Quote

Known more for his polish than his stuff in high school, Harrison has shown the ability to miss bats with three different pitches as he has matured physically. He generated a tremendous 41 percent swing-and-miss rate with his four-seam fastball in Double-A, sitting at 92-95 mph and peaking at 97 with riding action and a flat approach angle from a low three-quarters arm slot. Hitters didn't have much more success against his plus low-80s slider with sweep or his improved mid-80s changeup with fade and sink. 

Harrison debuted with the Giants in 2024, posting a 4.56 ERA in 124 1/3 innings pitched, and saw his overall swing-and-miss rates drop while he consistently struggled with walks. His three-pitch mix proved less potent against the best hitters in the world, and after spending time on the shuttle for the Giants in 2025 between Triple A and the major leagues, Harrison was part of the return in the Rafael Devers trade with Boston.

The Boston Red Sox Do Boston Red Sox Things
Boston is known for being an organization heavily focused on secondary pitches, and keen to limit fastball usage. Although the Brewers prefer to lean on the heater, they also like pitch diversification, so it's no shock that the two teams match up well on pitcher evaluations. Milwaukee got Quinn Priester from Boston last April, to patch their desperately injury-diminished starting rotation. Now, they've picked up Harrison, after a similar transformation during a similarly short time with the Sox.

Harrison's fastball usage shifted dramatically in his small sample of big-league appearances with the Red Sox, compared to those with the Giants:

His velocity dipped—perhaps from the long season, as we can also see his extension shortening after the trade. That's worth keeping an eye on. However, Harrison also made some fascinating deeper adjustments.

Transforming from a three-pitch mix and using four-seam fastballs 65% of the time to throwing just 46% four-seam fastballs and incorporating more slurves, cutters and sinkers made a big change to the quality of contact against Harrison. While his whiff rates weren't as strong, he was getting more chases and just a .280 xwOBACON (a statistic measuring the likely damage on contact), which would be an elite mark over a larger sample.

The changeup is particularly notable. After joining the Red Sox, Harrison changed his grip from a one-seam orientation into a kick-change, something the Brewers haven't seen much of within their player development system. This version usually creates more depth, but also more inconsistent movement, making it difficult to command at times. When it's right, the kick-change will produce negative induced vertical break numbers (IVB) with ease, and in Triple A, Harrison's approached -10 inches of IVB on occasion, with an overall three inches of extra drop on average.

The San Francisco Quandary
The Giants have recently had a number of high-profile, talented arms pass through their system with enormous hype, but have trouble translating that onto the big-league mound. Despite an elite defensive catcher in Patrick Bailey and a pitcher's park, it hasn't come together. Perhaps it's time to question their development processes.

One additional change Harrison made after going to the Red Sox lay in how he used his slurve. The Giants were adamantly and rigidly calling it to one side of the plate, down and away from lefties and toward the back foot of righties., The Red Sox encouraged him to open up both sides of the plate to complement his enhanced arsenal. The goal here was to elicit more confusion in the hitter's eye as to what pitch was coming his way, and the deception worked. He learned to land the pitch for strikes and backdoor it, as well as being able to bury it near the right-handed batter's box.

Harrison located the slurve in the zone more often, got more chase, and still maintained a higher quality of contact against the pitch than he managed with San Francisco in the first half of the year. He's never likely to be a strikeout artist in the majors, but his strong movement, funky delivery, and a wider pitch mix can make him a true all-around pitcher.

The majority of Harrison's Triple-A games came for the Red Sox, and they paint a picture of what he might look like at his best:

Harrison does have some walk issues. As with Priester, there might be a mechanical tweak in mind to clean up his delivery, but with his ability to garner chase, limit hard contact and miss bats both in and out of the strike zone, there is a lot to like about Kyle Harrison—especially in an organization like the Milwaukee Brewers.