Ronald Acuna Is Quietly Getting Better. And That’s Scary.

When the Los Angeles Dodgers take on the Atlanta Braves for a three-game series starting tonight, all eyes will be on a precocious young star who’s taken his game to the next level early this season. We are, of course, talking about L.A.’s Cody Bellinger … right? Certainly Bellinger has been great so far, and he was just named the National League’s player of the month for April. But “precocious young star” could also fit Bellinger’s counterpart on the Braves: Ronald Acuña Jr. Acuña’s ongoing development as a superstar hasn’t gotten as much attention as Bellinger’s, particularly during his recent slump and injury absence over the weekend, but Atlanta’s 21-year-old left fielder is showing important signs of improvement, too, despite surface-level numbers that can’t match those of his Dodger rival.

Acuña’s 2018 rookie season already put him in the history books. After being held down in Triple-A until April 25, he posted the 10th-best adjusted on-base plus slugging (OPS+) of any player age 20 or younger in a season since 1901,¹ edging out fellow NL East outfielder Juan Soto in the process, and he had the 14th-most wins above replacement² per game³ of any 20-or-under hitter over the same span, besting Bryce Harper, Ken Griffey Jr. and Willie Mays.

At a glance, Acuña’s early 2019 numbers actually represent a slight downgrade from that stellar rookie campaign. His OPS+ has dipped from 145 to 134, thanks to a big decline in slugging percentage (.552 to .487). Relatedly, Acuña is hitting home runs less frequently, and his isolated power is down nearly 70 points. It looks sure like a mild sophomore slump — albeit one we should all be so lucky to have.

Under the surface, however, Acuña has made some impressive strides this year in two important areas: plate discipline and defense.

Acuña’s 2018 walk rate of 9.2 percent was already better than league average — particularly impressive considering the whole “20-year-old rookie” factor — but his 2019 rate is up to 12.7 percent, which ranks 35th in baseball. He’s seeing more pitches (4.61 per plate appearance, second-most in baseball) and consistently getting into good hitter’s counts more often than he did a year ago. At the same time, Acuña has been whiffing a lot less so far in 2019. He struck out in 25.3 percent of his plate appearances last season, which ranked him around the bottom quarter of MLB hitters, but this year that rate is down to 21.8 percent, comfortably in the top half of the league.

Those changes give Acuña an overall strikeout-to-walk ratio of 1.72, which is the second-lowest in baseball for a hitter his age. Since a player’s batting eye is a core leading indicator for his underlying hitting skills, Acuña’s improved strike-zone judgment is a great sign of his ongoing development at the plate.

The other area in which Acuña has improved is defense. We combined the Ultimate Zone Rating figures found at FanGraphs and the Defensive Runs Saved numbers at Baseball-Reference.com and found that Acuña was no better than an average outfielder last season. (And he might have been worse than that — another prominent metric, Michael Humphreys’ Defensive Regression Analysis, considered him to be 3 runs below average.) This season, Acuña’s numbers are up no matter which source you consult; he’s on pace to have improved by about 10 runs — or 1 entire win — if he ends up playing the same number of innings as in 2018.⁴

The advanced fielding stats may just be catching up with what the eye test already knew about Acuña’s defensive potential. FanGraphs’ annual poll of fan fielding assessments had already judged Acuña to be one of the most talented left fielders in the game last season, thanks to his tremendous first step and speed to the ball off the bat. But now that he’s putting those skills to use in the metrics, Acuña’s potential value looks even greater: Per 162 team games, he’s tracking for 7.0 WAR this season, which basically matches what Ken Griffey Jr. and Albert Pujols had at the same age.

Acuña’s dip in power this year is a bit puzzling, particularly since his Statcast hitting metrics — including exit velocity, hard-hit ball rate and “barrels” (balls hit with the ideal velocity and launch angle) — are also down across the board. Some of that might come down to the injury that kept him out of Sunday’s game: “It’s been on and off,” Acuña told reporters through an interpreter after picking up four hits but exiting Saturday’s game early. “I’ve felt it for a while, so it wasn’t today. Today, it just kind of grabbed at me after I had a quick swing like that and got out of the box pretty quickly. That’s when I felt it. But I felt good coming into today, as always.”

But assuming his back isn’t an ongoing issue, Acuña should get plenty more pitches to crush as the season goes on, given his improved willingness to wait out favorable counts. Acuña may not be launching balls out of the park left and right like Bellinger at the moment — he hasn’t homered since April 16, in fact — but the foundation of his game still has superstardom stamped into it as much as ever.