Ronald Acuña Jr. had himself quite a rookie season last year. He finished with the most fWAR among National League rookies (in a tie with Juan Soto, 0.1 fWAR ahead of Harrison Bader) with 3.7 and took home the commensurate hardware at the season’s conclusion. He wasn’t the most productive member of his team, but still managed to place fourth (behind Freddie Freeman, Ozzie Albies, and Mike Foltynewicz) despite starting the season in Triple-A and missing a full month with a gruesome-looking leg injury suffered while dashing to first base at Fenway Park.

So, what’s a wunderkind like that do for a follow-up? Well, fast-forward to mid-September 2019, and Acuña is sitting on 5.1 fWAR with 12 games left to play. Pending something wild happening in the last seven percent of the season, he seems likely to finish somewhere between 5.0 and 5.5 fWAR, making him the most productive member of the team. (Josh Donaldson outperforming Acuña over those 12 games could also threaten this status, as Donaldson currently sits at 5.0 fWAR.) In at least one way, then, Acuña has ascended from a meaty contributor to the team’s MVP. But, how much has he really changed?

The answer to that question depends on a few things. For one, do you care about total production, or production on a rate basis? Acuña has clearly produced more this year, but is in a virtual tie with his 2018 self for production on a rate basis. For another, do you care about the bottom line, or are changes in components more interesting? If you only care about the bottom line, you can be satisfied with knowing that Acuña is still Acuña: he’s been really good, putting up All-Star-level play in both seasons of his young career so far. But, if you look at the component pieces, including the ones that feature differences of over half a win between 2018 and 2019 and are highlighted in the table above, some interesting differences start to take shape. Let’s take a look.

Ronald Acuña Jr., Outfielder

Coming into the 2019 season, I started maintaining a “list of interesting questions that we could see answered this year.” The list is populated with various baseball player- and team-related propositions, both related to the Braves and not. On the Braves-centric portion of the list, one of the first questions had to do with Acuña’s defense, owing to his relatively poor showing by defensive metrics during his rookie campaign. I worded the question this way: “Acuña’s defense — small sample size fluke, or [is he] actually a meh/bad defender?” The reason for this question? In 2018, Fangraphs’ use of UZR dinged his value to the tune of nearly 0.8 runs (most of a win) for his defensive play. Even if Fangraphs were using DRS, which was far more sanguine about Acuña’s defense in 2018, his overall defensive contribution (fielding relative to his peers at the positions he played, plus the positional adjustment for those positions) would be marginally negative, or below average relative to all position players.

Yet, in 2019, the defensive tables have turned. Positional adjustment aside (which has seen an increase due to more reps in center field), Acuña’s fielding has become above-average by UZR and experienced a similar increase from above-average to elite by DRS. The result has been defensive value that at worst ate into and at best did nothing for his overall production in 2018 transforming into a small-to-notable boost in value from defense in 2019. First, the bog-standard Fangraphs “Advanced Fielding” table:

There’s an interesting and easy narrative here, but the table blows it out of the water. When an outfielder makes a big gain defensively, you figure it comes from the outfielder’s bread-and-butter: getting under balls, especially potential extra-base hits, and catching him before they drop. For Acuña, though, that’s been anything but the case. In each of left field, center field, and right field, the “range” component captured by defensive metrics has gotten worse. This isn’t some kind of hinky defensive metric sample size thing, either, as the available Statcast defensive metrics and measures support this conclusion.

Across the board, relative to his outfielder peers, Acuña’s range has been worse. (Note that the average outfielder covers more ground with his jump in 2019 to 2018, which is the continuation of a trend where teams aren’t willing to punt defensive value, especially outfield defensive value, by plugging one-dimensional sluggers into those roles these days as much as they did so in the past.)

And yet, even with that dropoff in range, Acuña’s defense has grown and thrived. The reason? Well, hopefully it’s clear from the below.

(Note: the per-1,350 innings columns are not meant to suggest that these numbers are achievable over a full season, as they are not regressed. They are simply presented to give each partial-position-season slice equal footing for analysis. Acuña would not be a +21 arm runs right fielder if given the chance for a full season, and no one is trying to make this case.)

While there’s some tiny-sample-extrapolation silliness associated with huge numbers for 2018 CF and RF, where Acuña had barely any playing time, the green highlights make the disparity clear: the kid’s arm has gone from a notable liability to a fairly huge asset.

The run value of an outfielder’s throwing arm manifests itself in a few different ways. It’s not just about assists, it’s about the sum total of preventing advances on balls in play. Gunning guys down helps, sure, but so does simply preventing runners from trying to advance by virtue of existing. So does not throwing to the wrong base, or overthrowing the cutoff man (or everyone). So does taking better routes to cut balls off and prevent baserunners from advancing that way. It’s hard to say, without play-by-play review, whether the improvement in Acuña’s arm rating primarily derives from any one of these factors or a mixture, but the bottom line is that allowing extra bases for any reason has gone from a weakness to a strength, and this isn’t limited to his recent playing time in right field, either.

To examine how common erratic arm-based scores are, I compared the top 150 outfielders by innings in 2018 against their performance in 2019, by both UZR and DRS, and using both rate-based and non-rate-based measures.

As shown from these scatterplots, arm ratings are anything but stable, but they’re at least somewhat consistent in that the majority of outfielders either have consistently good arms, consistently bad arms, or average-y arms such that going from -1 in one year to +1 the next isn’t really that notable. There are only a handful of regular outfielders who went from a bad arm in 2018 to a good one in 2019, or vice versa. Among this set of outfielders, Acuña’s improvement in his arm rating is:

DRS, non-rate basis — tied for the best (Harper, Yelich);

DRS, rate basis — fourth-best, tied with three others;

UZR, non-rate basis — the best by a notable margin; and

UZR, rate basis — tied for the best (Brinson).

None of this is meant to be some kind of massive, forward-looking endorsement about Acuña’s increased defensive aptitude as an outfielder, or a high-certainty prognosis about his defensive value going forward. What it is, on the contrary, is a basic explanation for the relative surfeit in his defensive value in 2019. According to UZR, he’s added over a whole win with his arm in 2019 relative to where he was in 2018. According to DRS, the difference is a little more modest, but still in the same ballpark. I don’t know what this means going forward, but if you look back at that top line stat, where Acuña was worth 3.7 wins in 2018 and is up to 5.1 already this year, the improvement in his arm score stands as a big reason for the difference, more than enough to compensate for what looks to be a fairly problematic season for him, range-wise.

Lucky today, gone tomorrow

But wait! you might be thinking. You just showed me above that on a rate basis, Acuña was pretty much the same in 2018 compared to 2019. If the arm is such a big improvement, and it overcompensates for some range issues, why have his overall stats not budged? If you were indeed thinking that, great question! The answer is pretty simple, but not particularly gladdening or satisfying.

It’s writ large and clear on Acuña’s topline Statcast offensive outputs, as shown below:

In 2018, Acuña finished with a .388 wOBA that translated to a 143 wRC+, but it came on the back of a .373 xwOBA (around a 134 wRC+-equivalent). In short, he was a little lucky. We’re still talking elite inputs (that xwOBA is still above the 90th percentile), just results that were a little undeserved (more like 95th percentile results).

In 2019, though, the worm has turned, just a bit. Acuña currently sports a wOBA around .370 (.366 on Fangraphs, .372 on Baseball Savant, some weights are likely slightly off between the two), with an xwOBA around .010 higher. With the league-average wOBA increasing by .010 between last year and this year, his relative xwOBA hasn’t really changed, but with his wOBA falling, the result is lower production with the same inputs. Bummer. As a result, Acuña has a 124 wRC+ at this point, but his xwOBA suggests it should be more like 130 instead. If that were the case, he’d probably half something on the order of half a win more in his WAR than he does right now, which would combine with his improved arm rating to put some positive separation between his 2019 and 2018 production measures. As it is, though, that little bit of unfortunate luck is tethering him more closely to his 2018 outputs.

The thing is, this little unfortunate bit is frustrating in the grand scheme of things. Acuña has followed up his great offensive rookie season with either status quo or improvements, nearly across the board. Yet, the results haven’t quite been there to the same extent.

Despite a tiny drop in average exit velocity, he is barreling more balls than last year, in part due to better average elevation. The result is even better quality of contact than last year, even after accounting for the jump in the run environment.

His strikeout rate has jumped a little, but his walk rate has increased by even more.

While he’s whiffed more on pitches in the zone and also chased a bit more than last year, he’s also improved his ability to make contact and stay alive when he does chase, along with slight improvements in the recognition and swing rate of meatballs.

After a massive weakness associated with breaking balls last year, not so much associated with whiffing on them but rather rolling over them for easy outs, he has corrected that issue entirely, with fairly identical performance across the board in terms of pitch types. If anything, breaking balls have been his preferred pitches to hammer in 2019.

He’s reduced his grounder rate without increasing the rate of balls he gets under for unproductive contact. Overall, his rate of either barreled balls or well-struck ones that don’t quite meet the “barrel” designation has increased to the point where he manages one of these “solid or barrel contact” events about twice as often per ball in play as league average.

The point is, none of these developments really sound like they apply to someone who’s gone from a 143 wRC+ to a 124 wRC+, but that’s what’s happened. With the league’s offense improving overall, a drop given Acuña’s maintained performance isn’t surprising, but a nearly 20-point wRC+ drop seems steep, and it is. Folks expecting a 120s wRC+ output for him going forward may be underselling him a bit given the adjustments and improvements he’s made this season, even if the results don’t show it.

So, again — the top-line stuff is the same, the things underlying those, not so much. That’s probably always the case, as players can’t stay exactly the same, year-in, year-out. Even Khris Davis won’t be hitting .247 for a fifth consecutive season this year. But, there’s one thing that hasn’t changed: Ronald Acuña Jr. continues to be an absolute joy to watch, whether he’s hitting, fielding, throwing, or just goofing off. I don’t know if he’s going to pull together another 4.5 WAR/600 season next year, but I’m way more certain he’ll be just as entertaining in 2020 as he has been in his two major league seasons to date.