Ronald Acuna Is One of the Best Players in Baseball

The Braves have won plenty of games, and Ronald Acuna has hit plenty of homers. A common analytical writer trick is to open with an anecdote, to suck readers in before hitting them with statistics. I am a common analytical writer, but for this intro I want to focus on a Ronald Acuna single, leading off a game the Braves lost. And honestly, I don’t even care so much about the single itself. The Braves lost to the Cardinals on Monday. Acuna led off the bottom of the first with a ground-ball single off Miles Mikolas. The ball was hit well enough, but to understand what Acuna has already become, it’s most important to look at the process.

Acuna took a first-pitch strike. Happens sometimes. Especially leading off games. Mikolas throws an awful lot of strikes. Acuna was behind 0-and-1, but then he took a close fastball for a ball.

With the count 1-and-1, Acuna fouled off a fastball over the plate. Can’t connect with everything. As such, Acuna was behind 1-and-2, and hitters are always taught to protect with two strikes. Acuna took a very close fastball for a ball.

Mikolas had thrown the pitch he wanted, but Acuna didn’t offer. So, at 2-and-2, Mikolas decided to come back with a slider away. Acuna watched it spin by. Full count.

At that point, Mikolas came back into the zone. Acuna singled, getting enough of a pitch high and tight. It wasn’t a batted ball that’s going to show up on any Ronald Acuna highlight reels, but a plate appearance isn’t just about how it ends — it’s also about how it gets there. Acuna is showing increasing command of his at-bats. Acuna is now consistently in control, less than five months removed from his big-league debut.

Players aren’t supposed to be this good, this fast. We’ve been spoiled, and even in this year’s National League alone, we’ve got the 20-year-old Acuna and the 19-year-old Juan Soto. Out of the 196 players who have batted this year at least 400 times, Soto ranks ninth in wRC+, at 151. Acuna ranks tenth, at 150. There’s a spirited conversation going on right now about the NL Rookie of the Year. These aren’t even the only contenders. They’re just the two most visible ones. Soto and Acuna are likely to have extraordinary careers.

Acuna, obviously, has hit. He runs well, and he looks like a good defender. Acuna’s one of those players who could contribute in every single category. But there’s something else going on here, something that requires a slightly deeper examination. Acuna has grown over the course of the regular season. Though he got off to a hot start, he did cool down. Then he moved up in the lineup. His numbers all soared. The following table is completely ridiculous.

Ronald Acuna, 2018
Split PA K-BB% wRC+ xwOBA O-Swing% Z-Swing% Contact%
1st Half 184 24% 97 0.327 31% 67% 74%
2nd Half 253 8% 188 0.445 22% 70% 76%

As I say over and over again, I like to wait a while to see how rookies perform. I like to see how rookies do once the league starts to adjust to them. The league started to adjust to Ronald Acuna. He seemed to have a particular vulnerability to good fastballs. That vulnerability has disappeared. Pretty much all of Acuna’s vulnerabilities have disappeared. Acuna has adjusted back, and if you focus on that expected wOBA column for a second — before the All-Star break, Acuna ranked in the 48th percentile. Since the All-Star break, he ranks in the 99th. Ronald Acuna has already blossomed into a terror at the plate, and it all comes down to his approach.

In the first half of the season, Acuna swung at 31% of pitches out of the strike zone. That’s nothing awful, but it was still below average. Over just the past 30 days, Acuna has swung at 20% of pitches out of the strike zone. That ranks him 11th-lowest, out of 169 players. This is essentially the core of the whole story. Let’s talk about Acuna’s pitch selection.

I’ll be borrowing liberally from Baseball Savant. I’ll go back to comparing first-half Acuna and second-half Acuna. For this first plot, understand that Baseball Savant offers the ability to split pitches into three categories: clearly in the zone, clearly out of the zone, and on the edge. Here’s how often Acuna has swung at pitches in each category, expressed not as raw rates, but rather as percentiles. I like percentiles because they establish some league-wide context. Note that this is all in order from most swings to fewest, so that a low rank for out-of-zone swings is better.

Ideally, how would a hitter distribute his swings? The bulk of his swings would be against pitches in the zone. The rest would be against pitches on the edge. Between halves, Acuna has increased his in-zone swing rate by 24 percentile points. He’s become, in that sense, more aggressive. Yet he hasn’t become more aggressive across the board, because we see marked declines elsewhere. Against pitches on the edge, Acuna’s swing rate has dipped 23 percentile points. And against pitches clearly out of the zone, Acuna’s swing rate has dipped 45 percentile points. Acuna has seemingly learned how to stop chasing. It follows that his walk and strikeout rates have responded.

Now let’s focus just on chase rates — that is, swinging out of the zone. For this next plot, I’ll show you Acuna’s half-season percentiles in different counts. You see when Acuna is ahead in the count, when he’s even, when he’s behind in the count, and when he has two strikes.

You see that strong improvement across the board. Again, in this case, lower is better, since you’d prefer a hitter not chase very much. I’m most interested in Acuna’s improvement with two strikes, or when behind in the count. It’s easier for a hitter to appear more disciplined before the stakes are so high. As a hitter falls behind, there’s more pressure on his shoulders. That pressure hasn’t rattled Acuna’s approach at all. Since the break, Acuna’s had one of the very lowest chase rates when behind or with two strikes. That’s indicative of remarkable discipline.

What’s it all good for, anyway? Swinging at more strikes and at fewer balls should be obviously good on its face, but I can show you something else. Here are Acuna’s percentile ranks by count frequency.

Acuna, in the second half, has found himself ahead in the count far more often. In turn, he’s found himself behind in the count far less often. In the first half, he ranked in the 29th percentile in rate of pitches seen while ahead, and he ranked in the 66th percentile in rate of pitches seen while behind. In the second half, he’s ranked in the 84th and 8th, respectively. It’s meant 60 more pitches seen while ahead, and 57 fewer pitches seen while behind. Every batter everywhere is going to be more successful if he gets into better counts. You see better pitches to hit, and you don’t have to swing so defensively.

Something that happened for Acuna after the All-Star break is that he started batting leadoff. That might be a big part of it right there, similar to how we’ve seen Lorenzo Cain’s walk rate take off. But Acuna is also just standing in the box in a different way. Here you can see Acuna from earlier July, and Acuna from just the other day:

Acuna’s stance has become more closed. He also now has more of a crouch, and, maybe most significantly, you can see that he has lowered his hands. Moving a little forward in the sequence, the change in hand position remains visible:

There’s still the leg kick. And there doesn’t seem to be anything so different about Acuna’s actual swing. But over the past few months, Acuna has crouched lower while lowering his hands. His swing is therefore a little quicker to get going now, and there’s less movement in terms of his eye level. It’s important to understand everything that goes into what we interpret as plate discipline. Mechanical factors — as opposed to plain old decision-making — can determine whether or not a player swings out of the zone. If you don’t see the ball very well, your discipline isn’t likely to be good. If you have to decide whether to swing sooner, your discipline isn’t likely to be good. Acuna’s worked to become more balanced and quick, and as a consequence, he’s turned into someone both powerful and selectively aggressive. Which is the perfect kind of aggressive.

The present version of Ronald Acuna isn’t only a worthwhile Rookie of the Year. The present version of Ronald Acuna would make for a perennial MVP candidate. Because this is baseball, there’s no guarantee that the present version of Ronald Acuna will be the future version of Ronald Acuna. Any number of things might conceivably happen; careers can go awry with little notice. But we already knew before the season that Acuna was one of the most talented young players in the world. Now he’s become one of the most productive young players in the world, a player who can help his team in almost any facet. The money-maker for Acuna is going to be his bat, and he’s learned on the fly how to apply his magnificent swing to more of the right pitches. That was the only thing on the field that used to hold him back. Now there’s just about nothing left.

Jeff made Lookout Landing a thing, but he does not still write there about the Mariners. He does write here, sometimes about the Mariners, but usually not.

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Honestly, couldn’t think of anything better. Great article about one of the most exciting young players in the majors, on the team I love, so… yeah, like.


Very UnSullivan, he normally makes players we wouldn’t normally care about interesting.