Prospect Spotlight: How Jackson Chourio Turned Around His Season


Image credit: Jackson Chourio (Photo by Daniel Shirey/MLB Photos via Getty Images)

Jackson Chourio’s 2023 season is a story with two distinct halves. There’s the first half of the season where the Brewers outfielder swung and missed against offspeed pitches and chased fastballs out of the zone at a high rate. He looked overmatched at times.

Then the pre-tacked baseball that was a part of an MLB experiment during the first half of the season was shelved and the Double-A Southern League returned to the traditional baseball that was rubbed with mud before games. Chourio then seemed to immediately return to the form that made him one of the breakout stars of the 2022 minor league season.

It’s a wonderfully simple story, but does it hold up to deeper scrutiny?

Jackson ChourioBattingBB%K%ISOBABIP
Prior To July 14th.2497.0%21.4%.160.286
After July 14th.3187.1%15.2%.226.340
Jackson Chourio’s second half splits are a stark contrast from his first half.

Chourio seemingly found his swing overnight, putting up a torrid few weeks to finish July and start August, hitting .424/.480/.717 with an 11.8% strikeout rate in the first 21 games following the all-star break. Outside of a five-game mini-slump in mid-August, Chourio has been one of the best players in the Southern League in the second half.

As Baseball America reported earlier this year, pitchers increased their spin rates compared to their individual past norms when using the pre-tacked baseball. But even more importantly, the average pitcher added an additional two inches of carry on their four-seam fastball when using the pre-tacked baseball. A number of pitchers added three or more inches of carry.

While it sounds simplistic to say Chourio’s return to success can be explained largely by the return to the traditional baseball, a deeper dive into Chourio’s approach, swing decisions and contact rates makes it clear that this appears to be exactly what happened.

And it also raises bigger questions about the pre-tacked baseball. The hope with a pre-tacked baseball is that it will allow MLB to further crack down on pitchers using rosin to add grip to the baseball and ensure more consistency than is possible with baseballs that are rubbed up with mud at the stadium.

But if those baseballs move in ways that seem abnormal to hitters because pitchers get extra grip and spin, the unintended consequence may be not just swings and misses but also weak contact.

In Chourio’s case there was a clear improvement in approach with the traditional baseball. Chourio was making a nearly identical amount of contact but he was swinging significantly less at pitches out of the zone. 

Jackson ChourioMiss%Chase%In-Zone Whiff%Swing%
Prior To July 14th25%32%18.9%51%
After July 14th20%27%16.0%47%

Perhaps Chourio made an adjustment in approach and was knowingly swinging less at particular types of pitches. He’s letting the ball travel deeper in the zone during the second half of the season When looking at Chourio’s swings and misses from each half of the season it’s clear that high fastballs at the top of the zone or above were punishing Chourio in the first half. Fastballs are depicted in red on these charts, sliders are blue and changeups are purple.

With The Pre-Tacked Ball

With the Regular Baseball

The problem seems to disappear in the second half. Chourio’s miss rate didn’t drop against fastballs but his chase rate and swing rate dropped significantly. Subsequently his production against fastballs skyrocketed in the second half as he added over 400 percentage points of OPS. 

Jackson ChourioFB Miss%FB Chase%FB Swing%FB OPS
Prior To July 14th18%33%53%.822
After July 14th16%25%43%1.216

It’s reasonable to think that Chourio struggled to get on plane with fastballs in the first half, as those pitches weren’t moving in a normal fashion. Chourio would see a fastball and swing to make contact where he expected the pitch to be when it crossed the plate. But with the added ride that the pre-tacked ball gave pitchers, their fastballs would end up crossing the plate two to four inches above where Chourio expected

No hitter follows a pitch all the way to the plate with their eyes. To make swing decisions in time to hit a baseball, hitters must attempt to identify a pitch soon after a pitcher releases it and then try to extrapolate where that pitch will cross the plate. Hitters do this by using their experience of thousands and thousands of previous pitches to create patterns in their brain, a process called “visual chunking.” If a fastball thrown at a certain velocity moves in the same way as previous pitches that hitter has seen, then a hitter can not only time it properly but also line up the barrel of the bat to make contact with the baseball as close as possible to the bat’s sweet spot.  

With a pre-tacked baseball that had more movement than normal, Chourio and other Southern League hitters were effectively being tricked by their past experience. Chourio would see a fastball that looked almost exactly like fastballs he’d crushed before. But when this fastball got to the plate, it was crossing the plate higher in or above the strike zone than what was expected. That didn’t lead Chourio to swing and miss more, but it dramatically affected the quality of contact he was making and meant he was swinging at fastballs above the strike zone.

Jackson ChourioCHG Miss%CHG Chase%CHG Swing%CHG OPS
Prior To July 14th49%38%49%.578
After July 14th15%34%46%.668

This may have also led to a struggle against changeups, as the pitch had the most dynamic splits of any opposing pitch type between the two halves. Following the all-star break ball-swap Chourio cut his whiff rate against changeups by nearly 70% while seeing a nearly 100-point boost in OPS. Could this be a product of Chourio’s struggles against fastballs? Were changeups potentially moving in unusual ways as well? Whatever the culprit might be it’s clear there’s been significant progress by Chourio in the second half of 2023.

Jackson ChourioAvg. EV90th % EVMax EVBarrel %xSLUG
Prior To July 14th92.4106.6110.918.0%.334
After July 14th89.1104.5112.418.4%.427
While Chourio’s exit velocity data dropped in the second half his expected slugging skyrocketed.

Not only has the ball swap impacted Chourio’s plate skills, there’s been an unusual jump in production on contact in the second half despite a drop in exit velocity. While there’s been little difference in Chourio’s exit velocity or launch angle data, he’s seen his expected and actual power production skyrocket in the second half.  

It’s reasonable to assume that the pre-tacked ball not only impacted Chourio’s production against fastballs and changeups but also the quality of his batted balls of all types. In a game of inches a slight difference in where a hitters makes contact can be the difference between a hard-struck fly ball and a pop up. It’s possible he struggled to adjust to the physics bending effect of the pre-tacked ball and just didn’t hit as many well-struck drives at optimal angles. The issue is his barrel rate didn’t fluctuate between the two halves of the season, but the production on those barrels was wildly different from the first half to the second.

All this to say we’re still learning the impact of the pre-tacked ball. To know how many players experienced similar struggles against fastballs in the first half and a post all-star break surge in contact quality will take a fair amount of research, but we can clearly demonstrate that Chourio’s first-half struggles largely revolved around his struggles to handle a baseball that moved in ways he and other hitters hadn’t seen before.

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