No matter what happens during Trea Turner’s career on the field, he forever left his mark off the field by motivating Major League Baseball to change the way it handles trades of recent draft picks—the Trea Turner Rule, unofficially. Turner was drafted 13th overall by the Padres in 2014 out of North Carolina State, where he hit .342 with 113 steals in 173 games. After signing quickly for $2.9 million, he debuted by hitting .323 with 23 steals in 69 games between rookie-ball and low Single-A, establishing himself as a consensus top-100 prospect right away. And then that winter the Padres traded him. Sort of.

New general manager A.J. Preller was hell-bent on turning a 77-85 team into an immediate winner, giving up prospects, young major leaguers, and piles of cash to bring in Craig Kimbrel, Justin Upton, Wil Myers, Matt Kemp, James Shields, Melvin Upton, and Derek Norris, among others. Myers was acquired from the Rays in a three-team blockbuster that involved Turner being dealt to the Nationals. However, because he was just six months removed from signing and MLB rules prohibited draft picks from being traded for a full year, Turner’s inclusion in the swap had to be masked as a “player to be named later.”

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Everyone knew that Turner was the PTBNL—it didn’t even qualify as a poorly-kept secret—but in working that loophole the Padres had to keep him in the organization until 12 months had passed. And so he spent spring training in Padres camp and then began the 2015 season with their Double-A team, hitting .322 with 11 steals in 58 games while wearing the uniform and taking the coaching of his ex-team. MLB realized the absurdity of the situation, and with six weeks remaining in his mandated Padres stay they altered the rule for trading draft picks from 12 months to the day after the World Series, or about five months.

Turner was not grandfathered in, so he played out the string before finally being traded—officially—to the Nationals on June 14, 2015. By that time the Padres were already a mess that was only getting worse—the wreckage is still flaming to this day—and just 68 days later the Nationals called up Turner, who mostly struggled in 27 games at age 22. Turner started 2016 back at Triple-A, hitting .302 with 25 steals in 83 games before the Nationals called him up again in mid-July—not as a shortstop, but first as a second baseman and then as a center fielder.

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Turner was a revelation, hitting .342 with 13 homers and 33 steals in 73 games to finish runner-up to Corey Seager in the Rookie of the Year voting. He became just the fifth 23-year-old since 1970 to hit above .340 in at least 300 plate appearances, joining Albert Pujols, Joe Mauer, Robinson Cano, and Don Mattingly. Washington won the NL East with a 95-67 record, while San Diego tied for the worst record in the league at 68-94. Nationals general manager Mike Rizzo had somehow picked up a long-term building block in Turner—plus right-hander Joe Ross—in exchange for Steven Souza.

This offseason the Nationals traded Danny Espinosa to the Angels, clearing a path for Turner to resume playing shortstop after zero big-league starts there in 2016. He’s fared well, with 0.5 FRAA, but his bat has taken a big step backward from his otherworldly rookie showing. As of Friday, which was his 162nd career game, Turner was hitting just .262 this season and the lack of plate discipline that was tempting to ignore when he was hitting .342 is now an issue dragging his on-base percentage below .300. He’s been solid overall with 1.5 WARP in less than half a season, but the superstar light has dimmed a bit.

Turner’s approach at the plate and swing rates haven’t changed much from freshman to sophomore (again, through his 162nd career game, Friday):

YEAR O-SW% Z-SW% CON% BB% SO% 2016 31% 64% 78% 4% 18% 2017 28% 62% 78% 4% 19%

Roughly speaking he’s swinging at the same number of strikes and non-strikes, making the same amount of contact, and walking and striking out at the same rates. His walk rate ranks 218th out of 237 hitters with at least 500 plate appearances since the beginning of last year, but Turner isn’t quite as much of a hacker as those numbers may suggest. His out-of-zone and in-zone swing rates are actually lower than average, and his overall swing rate of 46 percent ranks 135th out of 237. Turner simply doesn’t swing that often, so how is it that he so rarely walks?

One big piece of that puzzle is pitchers throwing him a ton of strikes. Turner is one of just 28 hitters since 2016 to be thrown more than 50 percent pitches in the strike zone, and the majority of the other 27 are speedy, leadoff types with little power—predictable names like Ben Revere, Nori Aoki, Jarrod Dyson, Dee Gordon, Denard Span, and Billy Hamilton. Turner is certainly speedy and has been the Nationals’ leadoff hitter for nearly his entire career, but he also has 21 homers and 61 total extra-base hits through 162 games for a sturdy .185 Isolated Power.

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Of the 28 hitters to be thrown more than 50 percent strikes, only Mookie Betts—another young, slugging leadoff man—has a higher Isolated Power, and only Turner, Betts, Jason Kipnis, and Jonathan Lucroy are above .175. Pitchers have treated Turner as a prototypical leadoff hitter, as opposed to a really good hitter who happens to be batting leadoff. He doesn’t swing that often and perhaps could even be described as a reasonably patient hitter, especially for his age, but the sheer number of strikes he sees—and the number of opportunities he has to swing—makes drawing walks difficult.

In fact, of the 28 hitters since 2016 to see more than 50 percent strikes only Brett Gardner has a walk rate of 10 percent or higher. Gardner, not coincidentally, has the third-lowest swing rate in baseball, taking the bat off his shoulder just 36 percent of the time. He wills those walks into existence. Turner is never going to have Gardner’s extreme patience, but if he keeps showing above-average power, presumably pitchers will eventually stop throwing him so many strikes. Or maybe the Nationals will eventually move him out of the leadoff spot, which would likely have the same basic effect.

The much bigger concern is what happened to all of Turner’s hits. His strikeout rate has remained stable, so the number of balls he puts in play hasn’t changed much, but his batting average on those balls in play has plummeted from .388 last year to .302 this year. Of course, that explains how rather than why. Turner has not seen a major change in average exit velocity (89.0 mph last season, 88.2 mph this season) or in the number of balls he hits 95 mph or harder (38 percent last year, 38 percent this year). He’s hitting balls just as hard, so why (besides the inevitable regression) aren’t they dropping for hits?

Launch angle. Across baseball, hitters trying to add power by swinging up and putting the ball in the air more has become a big story, but Turner’s launch angle has declined from 10.8 degrees last season to 6.2 degrees this season. Last year his average launch angle ranked 156th out of 272 hitters with 200 or more at-bats to track. This year his average launch angle ranks 265th out of 298 hitters with 80 or more at-bats to track. Or, put another way: Turner has gone from a middle-of-the-pack launch angle—meaning lots of line drives—to one of the lowest launch angles—meaning lots of ground balls.

Via Baseball Savant: 2016 on the left; 2017 on the right

Grounders become hits at a higher rate than fly balls, which is why many speedy, low-power hitters still find success there. However, line drives become hits nearly three times as often as ground balls, and they often go for extra bases too. Compared to last season, Turner’s line-drive rate has dropped from 25 percent to 15 percent, his ground-ball rate has increased from 43 percent to 53 percent, and his “poorly hit” balls are up from 34 percent to 45 percent. In terms of exit velocity, he’s hitting the ball as hard, but instead of spraying tough-to-catch line drives he’s topping the ball or pounding that exit velocity into the dirt.

Turner is one of the fastest players in baseball and makes a decent amount of contact, so as we’re seeing this season he can have reasonable success (.262 AVG, .413 SLG) hitting tons of ground balls. However, because of his miniscule walk rate—thanks in part to pitchers throwing him so many strikes—his on-base percentage is below .300 and his overall production (.710 OPS) is lacking for a star. Turner has the speed to make a ground ball-heavy approach work enough to be a solid regular, but the rest of his skill set is too impressive to let that be his ceiling.

Turner is one of 310 players since 1950 to start at least 50 games at shortstop through age 24. Of those 310, only 36 did so while producing an OPS+ of at least 100. The first two spots belong to Seager (141) and Alex Rodriguez (138). Turner has a 112 OPS+, which ranks 17th between Manny Machado and Paul Molitor, and directly ahead of the amusing 109 OPS+ trio of Derek Jeter, Francisco Lindor, and … Khalil Greene. Anyway, you get the idea. Not many players hit as well as Turner while also playing shortstop so early in their careers.

Turner has racked up 5.7 WARP through 162 career games, which is the stuff of future superstardom, but on a per-game basis he’s been about half as valuable this season as he was last season. And it’s all from a decline at the plate, because his defense and baserunning haven’t fallen off. Turner has shown better at shortstop—his original position—than he did in center field, and he’s been successful on 85 percent of his steal attempts this season while swiping more bases (28) than anyone but Hamilton. Turner has 63 steals through 162 games, which is the eighth-most through 162 career games since 1950.

114 — Vince Coleman

102 — Tim Raines

78 — Gerald Young

70 — Juan Samuel

69 — Billy Hamilton

66 — Rickey Henderson

65 — Kenny Lofton

63 — Trea Turner

In terms of actual value, Turner’s baserunning—which includes those 63 steals at an 83 percent clip, but also non-steals stuff like tagging up and taking extra bases—has been 10.1 runs above average through 162 games, per BRR. (For some context, only Hamilton has reached double-digit BRR in a season since 2013.) He’s also grounded into just five double plays in 654 total plate appearances, a very low rate for a right-handed hitter. Turner is not only exceptionally fast, he’s good at turning that elite speed into on-field value for the Nationals.

Turner has the tools and skill set to thrive at all aspects of the game, but the question is whether the “real” him offensively is the 2016 line-drive machine or the 2017 worm-killing machine. Even to settle somewhere in between—matching his through-162-games line of .299/.333/.484, with adequate shortstop defense and elite-level baserunning—would make Turner one baseball’s best all-around players, but this season he’s trended in the wrong direction and it seems pretty clear that some adjustments are needed. It’s really a question of how many decades Padres fans will spend being furious about giving him up at 22.