Kyle Harrison's Red & Green Flags: Red Sox Analysts' Review of Brewers' Newest Acquisition

[Brandon Glick]

Image courtesy of © Eric Canha-Imagn Images

The Milwaukee Brewers' need for rotation depth certainly didn't portend a situation in which the team needed to remove a core piece of the roster, but that's exactly what happened when they struck a deal with the Boston Red Sox that landed left-handed starters Kyle Harrison and Shone Drohan (plus utility infielder David Hamilton) in Wisconsin. The price the Crew paid in exchange — Caleb Durbin, Andruw Monasterio, Anthony Seigler, and a top-75 draft pick — is hefty, particularly if you're not a believer in Harrison or his ability to rediscover the form that once made him one of the best prospects in all of baseball.

The 24-year-old southpaw first emerged as a top-100 prospect in 2022 in the San Francisco Giants' system, and after dominating High-A and Double-A hitters that year to the tune of a 2.71 ERA, Harrison became a consensus top-50 prospect. He maintained that status through the end of the 2024 season, when he exceeded his rookie eligibility limits while handling 124 1/3 innings for the Giants. His results were good enough on the surface; he managed a 4.56 ERA (4.33 FIP), 22.2% strikeout rate, 7.9% walk rate, and reasonably limited home runs and extra-base hits. But the under-the-hood numbers were ugly, from a 10th-percentile hard-hit rate to a simple inability to induce chases from opposing batters. A three-pitch pitcher, Harrison found little success with any of his primary offerings, with his slurve in particular getting lit up to the tune of a .347 wOBA.

A meager amount of playing time in the first half of 2025 led to Harrison getting shipped to Boston as part of the perplexing Rafael Devers trade. He mostly pitched for the Red Sox's Triple-A affiliate, Worcester, after the deal, though he did spend 12 successful innings in Beantown before the season was over.

"Harrison has a funky delivery where the ball comes out of his hand at a 27° arm angle. The comparable pitchers with that slot are Andrew Heaney, JP Sears, and Kyle Freeland. These low slot pitchers who have a solid fastball typically struggle to develop reliable secondary pitches. Harrison falls into the same boat, as he has crazy high fastball usage at 65%, while he only throws the slurve 25% of the time, and the changeup 10% of the time."
--Finley Rogan, Talk Sox, June 19, 2025 (Can the Red Sox Help Unlock Kyle Harrison?)

One of Harrison's long-running failings has been his inability to locate a secondary offering that plays nicely with his fastball. He's got good (but not elite) velocity, which has led to some calls in the past for a move to the bullpen.

Luckily, as our @Jake McKibbin explored following the trade, the Red Sox spent most of their limited time with Harrison tinkering with his arsenal. His fastball usage dropped dramatically, as he began to favor more cutters, sinkers, and slurves. The results were mixed (he generated fewer whiffs but was far better at mitigating hard contact), but the process is clear. The Brewers will need to find the right combination of pitches that'll keep Harrison effective multiple times through the order, but his primary red flag is already on its way to turning yellow.

"His changeup grip has been altered significantly to alter its spin rate. In his final appearance with the Giants, the changeup had an average spin rate of 2065 rpms; compared to his latest start with the Red Sox, that number fell to an average of 1253 rpms... The cutter appears to be the most changed pitch, having such a sharp horizontal break (three inches on average in his last start). It seems to be a weapon to challenge right-handed batters in with, being tossed to a left-hander just five times since joining the Red Sox organization."
--Nick John, Talk Sox, August 14, 2025 (Kyle Harrison's Tweaks Could Make Him A Key Figure in Red Sox's Rotation Down the Stretch)

One of the primary directives the Red Sox instituted when working with Harrison was improving his effectiveness against righties. Such hitters got to him for a .263/.326/.439 (.331 wOBA) line in 2024, and the disparity in his effectiveness between left-handed hitters and right-handed ones only grew wider last year. The aforementioned cutter and changeup were routinely tinkered with by Boston's pitching lab, as they tried to change the shape on both to allow Harrison to work both sides of the plate more effectively against hitters with the handedness advantage. He'll always rely on his fastball, but a reliable cutter and whiff-inducing change would go a long way to making him efficient against righties.

"One glaring positive from Harrison this season has been his command. He does not have enough innings to qualify, but Harrison’s 118 Location+ would rank in the top 20 among all pitchers in baseball. Yes, the sample size is small, and Harrison will likely see his command drop toward his career average Location+ (105), but good control paired with a high-spin fastball presents a more than viable profile to start games in 2025."
--Finley Rogan, Talk Sox, June 19, 2025 (Can the Red Sox Help Unlock Kyle Harrison?)

Command has long been one of Harrison's blue-chip skills. His career walk rate in the majors is just 8.0%, which is actually a notable improvement over his days as a prospect. There's been a trade-off, as his strikeout numbers have dwindled as well, but that feel for pitching (especially at such a young age) portends a bright future, even as he continues adding and reshaping pitches.

And, unlike the Red Sox and Giants, the Brewers have an exemplary defense, especially in the infield. Even if he never gets back to a strikeout rate above 30%, pitching to weak contact should be a more-than-effective strategy in Milwaukee.

"Harrison is still young — he won’t turn 25 until August — and the swingman role would be temporary until a permanent role in either the bullpen or the rotation is viewed as his final destination by the Red Sox. But for right now, it would be a waste to have him rotting away in Triple-A for the 2026 season when he could be providing support to both the rotation and bullpen by bouncing back and forth between the two."
--Nick John, Talk Sox, January 8, 2026 (Does Kyle Harrison Still Fit the Red Sox's Long-Term Plans?)

The Red Sox didn't surrender Harrison because they lost faith in his talent. After adding Sonny Gray, Johan Oviedo, and Ranger Suarez to a rotation highlighted by Garrett Crochet and Brayan Bello, they simply ran out of places to put the southpaw starter. He was superfluous in a system that also includes top left-handed pitching prospects Connelly Early and Payton Tolle, and the Brewers were wise to take advantage of that glut.

The same issue now faces the Brewers. Their starting five — currently comprised of Brandon Woodruff, Jacob Misiorowski, Quinn Priester, Chad Patrick, and Logan Henderson/Brandon Sproat — is in need of a left-hander, but it'll be hard to Harrison to overcome the established group and win a job out of camp. Likewise, with such a lefty-heavy bullpen, Milwaukee has no real incentive to plant their newest acquisition in the relief corps.

There's plenty of time to figure out how Harrison will factor into the mix, both for the 2026 season and beyond, but it's imperative that he doesn't get lost in the shuffle of so many other young starters if the Brewers hope to break even on this trade.

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[SF70]

Love the 2 options.

Stash him at AAA for the Brewers lab to finish-off the re-make. No rush. This team has so much SP depth it’s wonderful.