What's Wrong with Corbin Burnes?
The Brewers ace is struggling on all fronts so far this year. What's going wrong?
Introduction
In my top 50 player ranking, I had Corbin Burnes inside the top 10 and 2nd amongst all starters, citing his great consistency, complete arsenal of dependable pitches, and lack of any real weakness in his game. I also defended him for a tough stretch late in 2022, the first true sign of weakness Burnes had exhibited since the sticky substance ban in the middle of 2021. I stand by what I said then, but the Corbin Burnes of today is not that same paragon of consistency of 2021 and 2022.
Despite retaining a pitch mix virtually identical to last year’s in terms of usage and profile, his 8.8% walk rate is his highest since 2020, and his K rate has fallen 7% from a year ago to 23.5%. He has lost 1 mph or more on almost all of his pitches, but his three speed ranges remain distinct (cutter/sinker at 95, slider/change at 88, and curve at 80), so it shouldn't make too big of a difference. Of course, it'll reduce effectiveness somewhat, but a 6% drop in overall whiff rate, from 35% to 29%, indicates there’s more going on. The answer might be the pitch clock (he was taking 4.5 sec more per pitch a year ago), but it still deserves a deeper look.
Corbin Burnes' repertoire has struggled across the board this season.
The Problems
The main difference is in terms of execution, specifically against right-handed hitters. He's run reverse splits this season, remaining dominant against lefties, with regression in all three of his secondary offerings for righties. His curveball has dropped from an elite 20% swinging strike rate to a still solid 15%, his slider from an elite 25% to a still great 19%, and his sinker from a mediocre 9% to a pitiful 3.4%. Generally speaking, his secondaries also find the heart of the plate far more, leading to more barrels while eliciting fewer chases. More meatballs and more walks are the worst of both worlds.
Especially concerning is the sinker, which has increased in usage at the expense of the slider while also being thrown in the heart of the zone 21.5% of the time. This rate is almost as much as his elite cutter, and given opposing hitters are only whiffing 7.5% of the time when they swing at the sinker - 4x worse than the cutter it's meant to tunnel with - this change in usage is downright mind-boggling. The pitch that's intended to catch people off-guard while hunting for the cutter is the pitch people are crushing. Most of his pitches are seeing the heart of the plate more, making the walks and lack of chases even more concerning.
He's remained solid against lefties thanks to a curve that's stolen more strikes than ever, but it lacks the chase rate (just 26% now, compared to 33% a year ago) that made it devastating. The changeup saw a similar decrease in whiff productivity but continues to limit hard contact for the time being. The changeup has seen a significant uptick in swing rate, so people pick it out as a pitch they want to attack. Time will tell if the weak contact it currently elicits holds.
PLV finds his cutter to be significantly worse (it was graded at ~3.2 PLA in 2022), but his other pitches remain solid. It continues to hate his sinker, which is now his 2nd most-used pitch against RHB for reasons I don't understand.
Lastly, his cutter and money pitch. It only lost 0.5 mph, but a 1.5-inch loss in horizontal break is costly. It finds the zone at a higher rate than before, but the walks arise from much more uncompetitive misses - a 12% drop in chase rate coincides with a 3% increase in walk rate. Nobody is fooled in the zone either - an 8% increase in flyball rate and a 5% increase in barrel rate has meant that he's already matched the home run total on the pitch he had in 2022 while not putting people away at nearly the same rate. He's gotten more called strikes but far less swinging - a serial trend, and probably a result of that loss in break. While Corbin can shuffle between his secondaries as he pleases, the cutter has to be there. It's his pitch.
Conclusion
I stand by my belief that his poor August stretch in 2022 revealed few issues inherent with Burnes’ repertoire - outside of maybe the cutter with its location, every pitch in the repertoire was in reasonably working order. This is different: he’s locating worse, not inducing as many chases on any pitches, allowing more fly balls, losing velocity (albeit likely from the pitch clock), and making questionable pitch mix decisions against the platoon side he’s struggling against.
My instincts say that the problem is simply pitch clock adaptation and game-planning, and the issues may not extend beyond the cutter otherwise. Pitch modeling still is a fan of all of his secondary pitches; PitcherList’s PLV as shown before, as well as Eno Sarris' Pitching+ model believe that only the cutter has lost its effectiveness (presumably from the loss of 1.5 inches of horizontal break). I believe it's a little more than that, but it can be said that his cutter is the foundation of his entire repertoire. One could argue that the five starts with the worst cutter performance by CSW% were precisely his five worst starts this year (3/30 @ CHC, 4/5 vs. NYM, 4/17 @ SEA, 5/22 vs. HOU, 6/19 vs. ARZ).
Corbin Burnes will get back on track. He has five usable pitches with a tried and true foundation. It's all mechanical and fitness: can he get the cutter back? Can he avoid the long, arduous outings that cost him command and incentivize hitters to chase less? His ERA is around 4.00 now, but I would imagine he gets back to 3.30 territory quite quickly; outside the Dbacks game (which was one bad 1st inning), he hasn't had another disaster for a month, in large part due to a resurgent string of performances from his cutter. If the cutter returns in force, the rest of the arsenal will follow.
Sources:
PitcherList
Alex Chamberlain’s Pitch Leaderboard
FanGraphs