Grant Hartwig still has work to do

By Mets relief prospect standards, Grant Hartwig’s 2023 was a huge success in that he ended it with his elbow intact and pitched to an ERA below six. That’s a low bar to clear though, and Hartwig’s season line - 4.84 ERA, 103 DRA-, 7.82 K/9, 9.5 K-BB% in 35.1 innings - leaves a lot to be desired. His line in Syracuse wasn’t much better, making this a bit of a let down on the whole.

Hartwig entered the year with some significant momentum. A UDFA signed in 2021 whose college career was interrupted by Tommy John surgery and COVID, the right-hander impressed as he flew up the minor league ladder in 2022. He posted 1.75 ERA in 56.2 innings across four level while striking out 83 and keeping his walks in-check enough to have success. An ERA of zero through four innings with four strikeouts in 2023 spring training only added to the excitement, and many (including the Amazin’ Avenue minor league team) began talking about Hartwig as someone who could seize a key role in the Mets bullpen in short order.

Unfortunately, access to major league data after Hartwig’s call up poured some cold water on that optimism pretty quickly. Neither PitchingBot (42 Ovr, 50 is average) or Pitching+ (93, 100 is average) were particularly fond of Hartwig’s stuff or command profile. His sinker - the pitch he threw most frequently - ranks especially poorly, and was hammered for a .384 wOBA (equivalent to the mark Bryce Harper posted in 2022). He lacks high-end velocity, extension, spin rates, or spin deviation. It’s just a very vanilla arsenal at present.

Exacerbating this issue is Hartwig’s actual pitch usage. Like so many pitchers in the organization (cough David Peterson cough), the Mets have Hartwig adhering to an outdated strategy focused on establishing the fastball, learning a changeup, and using breaking stuff only sparingly. Hartwig’s sweeper is far and away his best pitch - his only above average offering by both PitchingBot and Pitching+ - and its above average drop makes it at least unusual (though not a true outlier) from a movement perspective. Correspondingly, his sweeper held batters to a .236 wOBA, the 25th best mark among pitchers with at least 100 sweepers thrown. This a pitch Hartwig should be throwing 40-50% of the time rather than the ~30% clip it’s currently deployed at.

It’s also clear that Hartwig needs a different fastball, even if that pitch becomes secondary to the sweeper. The standard move here would be to teach him a cutter, and he already throws one that actually worked pretty well last year (.271 wOBA against w/ 99 pitches thrown). Both PitchingBot and Pitching+ hate the pitch while actually preferring his four-seam fastball (which got absolutely hammered in an extremely small sample) which is perhaps reason for pause. On the other hand, the pitch has a very unusual lack of horizontal movement which might make it enough of a wrinkle to fool batters.

Interestingly, this usage shift seems to be something Hartwig has already adopted, but only against left handed hitters. Below you can see Hartwig’s usage split by batter handedness:

Alex Chamberlain Pitch Leaderboard v5

While he threw only 25 cutters against right handed batters, Hartwig’s cutter was his most used pitch against lefties and induced a .177 wOBA. He still threw too many sinkers and sidelined his sweeper too much, but upping the usage on the cutter was a step in the right direction and could go a long way towards explaining his significant reverse platoon splits; righties hit a robust .357 wOBA, while lefties tallied only a .261 wOBA.

Small sample size caveats are abundant here of course, as they always are when discussing relief pitchers. Righties also absolutely shellacked Hartwig’s cutter to the tune of a .610 wOBA and an average exit velocity over 95. It’s an especially small sample at only 25 pitches however, and my contention is that he’d be better off running with this newer, somewhat unknown pitch as opposed to his sinker, for which there’s much more extensive evidence of being a non-viable offering.

What sorts of improvements could Hartwig make to his cutter? The most critical one is probably location-based, as he threw far too many cutters over the heart of the plate last season:

Baseball Savant

Unsurprisingly, that led to less than ideal results:

Hartwig’s control isn’t exactly fine (evidenced by his walk rate) but he’s demonstrated an ability to spot his other offerings more effectively. Perhaps more experience with the cutter will allow him to do the same. He could also look to work drop into the pitch, making it more similar to Robert Stephenson’s plus offering (68 PitchingBot, 99 Stuff+). Neither of these are necessarily easy improvements to implement, but they’re hardly impossible and could give Hartwig a much better fastball to use against batters from both sides of the plate.

Hartwig is not someone that the Mets should be relying on in any major way going into 2024, at least not based on his 2023 numbers, nor is he someone that really has the upside to become anything more than a 7th inning arm in the long term. There are clear tweaks that can be made for him to improve, however, and he can still serve as a useful optionable depth arm if additional coaching can further optimize his arsenal.