Jackson Chourio hasn’t taken the leap that many expected this season. After a slow start to 2024, he posted a 143 wRC+ and a .303/.358/.525 slash line from June through the end of the season. Entering his age-21 season, there was an expectation that he was near superstar status. Instead, it’s been a productive but not otherworldly season: a 115 wRC+ and a .274/.309/.468 slashline. Add his solid centerfield defense to the mix, and Chourio is still the most valuable player by fWAR on the MLB-best Milwaukee Brewers.
Chourio’s approach explains the up-and-down performances: he’s uber-aggressive with a good, not-great batted ball profile. When things are going well for Chourio, like the second half of last year, the swing aggression turns into both hits and the occasional home run. When they’re not, he’s unable to outslug his struggles. Out of the gate this year, Chourio was far more aggressive than we had seen in year one.
He hit well early in the season, putting up a 116 wRC+ in April, but it was in an unsustainable fashion. He hit .271 with a meager 1.5% walk rate, somehow finding a way to be an above-average hitter while having a sub-.300 OBP.
He had a 58.6% swing rate in April, which dropped to 52.3% in June, as his swing aggression tapered off from the first month of the season. In April, those swings uncharacteristically made contact with fastballs and missed offspeed pitches. 
In April of this year, his whiff rate against fastballs was near a career low and a near high against offspeed. He continued a relatively high whiff rate against breaking pitches, but it looked like a potential approach change was underway to start the season. It has since faded, as Chourio continues to only feast on the slow stuff. Through almost two full seasons, Chourio has established himself as a slowball hitter.
Despite more whiffs, Chourio hits the slower stuff exceptionally well. He can make a good amount of contact despite poor decision-making, and then can feast on breaking pitches that are left too far in the zone.
It may seem unsustainable, but Chourio gets more non-fastballs in the zone than the league average. Unsurprisingly, he hits them pretty well.
The problem with being a slowball hitter is that this is still a fastball league: nearly 60% of the pitches Chourio sees are fastballs. Chourio also has a massive hole at the top of the zone, effectively creating an easy path for pitchers to get something past him.
Chourio’s swing path is a decently flat 27 degrees, below the 32-degree league average. In theory, a flatter swing would help deal with high pitches, since it would be easier to get on plane with a flatter swing path. However, Chourio’s 28.4% whiff rate on high fastballs ranks in the 35th percentile on swings with a 27-degree or flatter tilt. He’s getting swings off that should be capable of contact, but are not. Chourio’s 106 contact ability is above-average according to PLV, so this does not manifest as an overall contact problem.
Naturally, I wondered if there was an issue with velocity. With a flat swing and aggressive approach, the one thing that could put the ball past him would be velocity. Anecdotally speaking, I had seen a lot of “overmatched” swings from Chourio.
The data doesn’t agree, though. 141 hitters have faced at least 750 pitches this season, and only 30 have a better xwOBA on fastballs above 95 mph compared to fastballs below 95 mph. Chourio’s .016 xwOBA improvement on fastballs above 95 mph ranks as the 16th-biggest overperformance. His overall performance against fastballs is in the bottom 20 of the group, so it’s not an edge that he has, but rather the lack of a weakness.
Instead, Chourio suffers from sitting on secondaries while being an aggressive hitter. An example of this comes in a recent at-bat against Matthew Boyd. Boyd starts the at-bat with a fastball that gets looked at for strike one and then misses low-and-in with a changeup to level the count. He then gets Chourio with a beautiful backfoot slider to get to two strikes. At two strikes, Boyd only throws the fastball 22% of the time, with no other pitch seeing more than 30% usage. He goes to the fastball, which Chourio is very clearly late on.
This persisted throughout the season: Chourio isn’t necessarily equipped to be in the driver’s seat in pitcher’s counts. His reluctance to hit fastballs puts him on the back foot when he gets to two strikes, where he’s able to make contact but not in a productive manner. While the fastballs still get the better of him, he’s drastically improved his performance in pitcher-friendly counts. 
He’s cut his chase rate down from where it has been all season, with his July figures sitting closer to his average from last year. Better decision-making on pitches in the zone also had a direct effect on his decision-making in the zone, where he faced more hittable pitches when he laid off the junk. Chourio even set his max EV for the season on a fastball in a pitcher-friendly count, coming on a sinker that he laced back up the middle.
Chourio had a 179 wRC+ in July before getting hurt, the best single-month wRC+ he’s had so far in his big league career. His skills when he is ahead in the count are very clear, but the leap in performance while behind in the count is what’s driving the surge in performance.
Chourio’s approach is what holds him back from reaching the highs of superstardom: he possesses great swing speed, solid bat-to-ball ability, and consistent attack angles. When he’s sitting back against fastballs, while still being aggressive, his performance lags across the board. When he returns from injury, the Brewers will look to him to pick up where he left off in their battle for the NL Central crown.
