Stat This, Not That: How to assess whether April breakouts from Jarred Kelenic, Jorge Mateo and Brandon Marsh are for real

Give a man a fish, they say, and you feed him for a day. Teach him to fish, and you feed him for life. Trying to decipher the meaning of MLB’s April stats is similar. Provide your opinion on whether Jarred Kelenic is finally breaking out, and you’ll satisfy a fan for today. But teach that fan to monitor the most revealing numbers, and they can check on Kelenic’s impending breakout every year.

I kid, I kid — mostly. The Seattle Mariners’ highly scrutinized outfielder is one of this season’s early leaderboard standouts. Through 17 games, he’s batting .310/.385/.603 with four homers and improved strikeout and walk rates. He’s one of a few young or young-ish hitters inspiring eyes emojis with a great start that feels like it could be the beginning of something bigger.

Using the park- and era-adjusted wRC+ metric at FanGraphs, we can take a snapshot of the league’s top hitters so far, entering Friday's action. Now, no one is going to be 140% better than league average over a full season (Aaron Judge’s monster 2022 checked in at 207 wRC+, or 107% better than average), but some of these names will in fact stick.

1. Matt Chapman, Blue Jays third baseman, 239 wRC+

2. Brandon Marsh, Phillies outfielder, 196

3. Luis Arraez, Marlins second baseman, 192

4. Anthony Rizzo, Yankees first baseman, 192

5. Jorge Mateo, Orioles shortstop, 191

6. Brandon Lowe, Rays second baseman, 190

7. Jarred Kelenic, Mariners outfielder, 181

8. Yandy Diaz, Rays infielder, 180

9. Brandon Nimmo, Mets outfielder, 179

10. Kyle Tucker, Astros outfielder, 177

We’re going to focus on the newest and least proven members of this fleeting club, the bolded names who might or might not be fulfilling prospect promise after less-than-smashing beginnings to their major-league careers. Clearly, the results in 2023 so far are great. But how can we tell if that success is based on real, sustainable changes?

“Small sample size” is the longstanding bane of early season enthusiasm, but the refrain can be a simple nudge to dig deeper, rather than a total killjoy. Nothing a baseball player does in 50 or 100 plate appearances should transform long-term expectations, but there are distinctions in how one lands on the leaderboards that can inspire optimism or foretell a crash back to earth.

Those hints can be found only by digging beneath the surface of the typical statistics — batting average, on-base percentage, homers — that we might use to gauge full-season success. The best kernels of truth, at this stage of the season, are found in two main categories: plate discipline (deciding which pitches to swing at) and quality of contact (how hard you’re hitting the ball and to where).

Let’s see how each April breakout fares.

Brandon Marsh

The wet-haired, bushy-bearded Marsh joined the Phillies in a trade last summer after sputtering as a young player with the Los Angeles Angels. So far this season, he has been collecting extra-base hits left and right — 3 homers, 3 triples and 6 doubles. He leads MLB with an absurd .514 batting average on balls in play (BABIP), which is typically the first horseman of apocalyptic regression.

Plate discipline: Marsh has been swinging less, which isn’t a straight line to success but often has benefits. Ideally, the change takes the form of swinging at fewer pitches outside the zone but still attacking the pitches you can mash. Marsh’s changes haven’t taken exactly that form — he has shaved more points off his zone-swing rate than his chase rate — but they do reflect an awareness that he’s much better against fastballs. He’s swinging at more fastballs than ever while laying off more breaking balls and offspeed offerings. His contact rate — simply the percentage of his swings that make contact — has gone from worrisome to average, but his strikeout rate is still very high at 31.8%.

Quality of contact: The biggest reason to believe in Marsh might be his hitting coach. The Phillies’ Kevin Long has successfully retooled a parade of hitters and has already made visible changes with Marsh. Since the trade last summer, Long has helped Marsh simplify his pre-pitch setup to improve his timing. That probably deserves the credit for his increased rate of contact and for some marginal improvement in how hard Marsh is hitting the ball. But while more contact is certainly better, it’s not likely to be this much better. Marsh, a blazing runner, has made some serious hay on ground balls, but that simply can’t continue like this. So far this season, he’s hitting more grounders than usual and fewer line drives, which isn’t a recipe for sustained success.

Jorge Mateo

The 27-year-old Mateo bounced around for years after the prospect sheen that made him a key piece in the Yankees’ 2017 trade for Sonny Gray wore off. Finally, in 2022, he got consistent playing time in Baltimore. He stole 35 bases and played a very good shortstop but logged the third-lowest on-base percentage of any hitter with 500 plate appearances.

Plate discipline: Like Marsh, Mateo is swinging less so far this season. His adjustments, however, appear more targeted and provide a model for how decisive changes can alter a hitter’s outlook fairly quickly. In Mateo’s first extended run of playing time, some patterns became apparent: He struggled with low pitches — both in the zone and diving out of it — but thrived against pitches up and in. His approach this year has obviously been tailored to those tendencies. He’s swinging at only 27% of pitches in the lower third of the zone or below, according to Statcast, after hacking at 43% last season. In his nitro zone, meanwhile, he’s still going after about 77% of pitches.

His strikeout rate is down to 15.8%, and his once-minuscule walk rate is now close to league average. Baseball Prospectus’ DRC+, which uses statistical modeling to cut through noise and zero in on a player’s “deserved” performance, has Mateo’s 2023 line at .270/.340/.438. Over a full season, that would amount to a less powerful version of Francisco Lindor’s 2022, which would make Mateo a bona fide breakout star when paired with his speed and defense.

Quality of contact: Mateo has already set a personal-best maximum exit velocity this season, which is a sign of more confident, propulsive swings. It also appears that he has done a better job of keeping his front shoulder and hip closed early in his swing to create more power. He doesn’t need to be swinging for the fences, but rising into the upper tier of baseball in hard-hit balls per swing — which he has done — is a great formula to maintain a higher level of performance.

Jarred Kelenic

Since he was the Mariners’ prize in the deal that sent Edwin Diaz and Robinson Cano to the New York Mets, Kelenic’s struggle to launch has made him a point of intrigue on both coasts. His batting lines have been truly bad in two partial MLB seasons — 93 games with a 74 wRC+ in 2021, 54 games with a 55 wRC+ in 2022 — but no 23-year-old should be written off over that. And certainly not 23-year-olds capable of belting home runs 482 feet.

Plate discipline: Kelenic’s sky-high MLB strikeout rates have been particularly confounding because he didn’t have the same issues in the high minors. The difference seems to be an inability to handle big-league breaking and offspeed stuff. So, like Mateo trying to lay off pitches in his cold zones, Kelenic has cut down on swings at non-fastballs — from a 47.3% swing rate in 2022 to 38.5% in 2023. Meanwhile, and perhaps more importantly, he’s taking more cuts at fastballs in advantageous counts, despite swinging less overall. That’s a positive, self-aware move, even if his continued ineffectiveness when he does have to hit a breaking ball raises some concerns about how pitchers might adjust.

Quality of contact: Perhaps caught in between or trying to hit for too much power, Kelenic popped up an alarming 15.5% of balls with which he made contact in 2022, which amounted to automatic outs on top of the high strikeout rate. This season, his mix is much healthier and more in line with what prospect evaluators expected. He’s lacing line drives and hard fly balls regularly.

Again, there’s a disparity between fastballs and everything else. Kelenic's average exit velocity against heaters is a terrific 95 mph. Against everything else, he’s at 85.3 mph. If he can keep his approach humming to where there are enough fastballs in his diet to get hits and reach his power potential, he’s going to be a great hitter (if less otherworldly than he has been this month). Even if he starts facing more breaking ball challenges, his rhythm against fastballs looks like enough to keep him afloat as an average or better hitter. That said, there's still an element of risk involved in his apparent weakness and in the knowledge that it has brought him down before.