Spoiled Milk in Your Fine Wine

Let’s go back to July 14th, Oneil Cruz of the Pittsburgh Pirates is shining in the Home Run Derby in Atlanta:

There’s never been a doubt about Oneil Cruz’s tools, he hits the ball harder than LITERALLY anyone, he even throws the ball harder than LITERALLY anyone:

If there’s a projectile that needs to be moved at super sonic speed, there’s probably no one better to call than Oneil Cruz (maybe a hammerthrow gold medalist). At the same time, Oneil Cruz is a projectile blasting god but not a baseball god, because that title belongs to Shohei Ohtani:

It’s no secret why Oneil Cruz is not a baseball god, all those blue numbers that surround his gaudy 100’s on his percentile rankings. While his contact and discipline issues have been analyzed time and time again, what made this season particularly interesting was what came before it. In 2024, Oneil Cruz had what many thought to be a “breakout” season. He showed to be a competent centerfielder at a gigantic 6’7”, and posted an above average 110 wRC+ and 3.6 fWAR laying the foundation for what we thought would be the next step in 2025 for his Age-26 season.

Player development is in no way a linear curve. Not all players continue to stack improvement after improvement the way Bryce Harper or Ronald Acuna Jr. did. Even then, there are underlying assumptions we make about a player’s growth as they mature, the biggest being that a player’s plate discipline will improve as they age. This is the oldest basebally scouting wisdom you hear, “player X can hit for power, but whiffs too much, but as he ages, he’ll have a better sense of the plate, and player X will soon be All Star X!” We place these expected growth without any confirmation of training, coaching or changes in approach. Simply the expectation that the discipline wine will age in the bottle.

Now lets get back to Oneil Cruz. While 2024 was a good season for Cruz, his 30.2% K-rate was the 4th highest among qualified hitters, and 8.5% walk-rate was only slightly above average. The scout inside you says, “Cruz will improve on this, because wine” so let’s review his 2025 stat line:

So while he improved his walk rate, overall it was a disaster season for Cruz. Even though he shined during All Star Week at the Home Run derby, the player Oneil Cruz is yet to be selected to an All Star team. What happened? The fine wine season turned into spoiled milk. To really understand the differences between 2024 to 2025, let’s look at the standard counting stats:

There’s a few goods here, he walked more and hit 20 HR despite getting 55 fewer plate appearances, but the biggest thing that stands out is the near 50 hits that vanished from his hit totals. Obviously part of that is having fewer plate appearances, but also he simply was not putting the ball in as much. In 2024 he put 364 balls in play (about 61% of his PA) but in 2025 he only put 302 balls in play (56% of his PA). That combined with a big drop in BABIP (.347 to .262) meant he was both putting fewer balls in play and getting less hits when he did put balls in play.

This is the spoiled milk that shows up when a player “matures” and improves their plate discipline. As you chase less, swing and miss less, it doesn’t mean you’re going to hit the extra balls with the same quality. To show this, here’s a graph of player-seasons with +500 PA from the past two years, with the x-axis showing how often a player ended his plate appearance with a ball in play, and the xwOBACON (xwOBA on contact) that the balls in play had:

On the left top is Ohtani and Judge, who only put the ball in play less than 60% of the time, but the ball is absolutely crushed. On the right bottom are Jacob Wilson and Luis Arraez, who are not crushing the ball at all, but make up in volume, by putting the ball in play almost all the time. In between you can see the trade off that occurs.

This tradeoff that occurs with maturation is hardly exclusive to Oneil Cruz. In fact, if you aggregate player seasons by age, and measure plate discipline you can numerically observe this. To do this, I’m using the plate discipline estimator of [ O-Swing% - 2* Z-Swing% ] ; This is taking the percent of times you swing at a pitch in the strike zone, and subtracting the percent of times you swing at a pitch outside the strike zone (multiplied by two). I looked at all players between ages of 20 to 30 with minimum 100 PA between 2015-2025 and averaged the player’s Discipline Estimator against BABIP (I couldn’t readily get xwOBACON for this data):

There’s a little bit of small sample funnyness at Age-21 since there are not many players who debut that early, but for the most part you can see that a hitter’s discipline does improve with age. At the same time, you can see the BABIP begin to decrease as the quality of balls put in play goes through the trade off.

So what’s the point in all this? The answer is that while the game of baseball is set of independent events, the underlying skills and characteristics are all interconnected. A player with good contact skill but slow swing can’t just turn a knob that says “swing speed” and expect to improve without affecting another part of his game. Similarly a pitcher can’t push the “more stuff+” button without sacrificing an aspect of his arsenal. A player missing one tool isn’t simply one tool away from being an MVP like Thanos missing a Mind Stone.