Just 10 months ago, Bryce Harper was putting the finishing touches on one of the greatest position-player performances of all time. Harper slugged his way to 9.5 wins above replacement last season, largely on the basis of the best single-season offensive campaign since Barry Bonds broke baseball in 2004. But this year, Harper’s hitting numbers have slipped, and his overall value has suffered as a result.

Sometimes drastic drop-offs like these are the result of bad luck, or even opposing pitchers changing their approach. But with the help of data from MLB’s radar-tracking Statcast system, we can see that Harper’s slump is a consequence of diminished power, which might be a more difficult fix.

Harper’s eye-popping .330/.460/.649 triple-slash line in 2015 was driven by patience and power. He walked 0.95 times for every strikeout, the sixth-best ratio in baseball, and led MLB in isolated power (ISO) with a .319 mark. Between intentional walks, Harper managed to drive 42 home runs, making monster blasts a regular occurrence in Nationals Park.

Some of that magic carried on into the first half of 2016, but lately Harper’s stats have taken a tumble. Although his walk-to-strikeout ratio has improved, his ISO is down more than a hundred points, to .209. As a result, his overall production has dropped to a pedestrian (at least by Harper’s standards) 115 Weighted Runs Created Plus (wRC+), from 197 the year before.

Statcast has tracked Harper’s decline in real time. It monitors the exit velocity and launch angle every time a player makes contact, characteristics which can be used together to predict a batted ball’s value in terms of runs. By looking at changes in Harper’s underlying Statcast metrics, one can judge whether he has been merely unlucky, or if he’s truly experiencing a decline in skills.

In 2015, the actual value of Harper’s batted balls exceeded what we would have expected, given their exit velocity/launch angle combinations, by a significant margin. Given available data, we can’t say what combination of foot-speed and luck drove that divergence, but so far this year his actual production on batted balls has declined precipitously — even going below what the model predicts.

More tellingly, Harper’s predicted batted-ball value has fallen substantially as well. Much of that reduction is owed to a decrease in exit velocity, one of the biggest components of hitting skill. Hit the ball harder, and it’s more likely to go past fielders or over fences; softer, and those same fielders can throw you out, or a would-be home run turns into a warning-track flyout instead.

So far in 2016, Harper’s exit velocity is down almost two and a half mph from his average last season — even as MLB’s overall exit velocity has spiked this year. Alongside that decrease in exit velocity has come a sharp increase in launch angle. Harper’s typical batted ball in 2015 ranged between a launch angle of about 13 to 16 degrees, giving him a line drive swing conducive to a high batting average. Harper started this season in a similar range, but he’s seen his game-by-game average creep up to a 20 degree launch angle in the last few weeks. Pop-ups have come along with that elevated launch angle, as he’s seen his infield-fly percentage almost double from 5.8 percent in 2015 to 10.7 percent in 2016. That subset of his batted balls are certain outs, so they explain a significant fraction of the decline in his batting average on balls in play.

And it’s not a product of pitchers approaching Harper differently this season. Usually, balls thrown higher in the zone tend to get hit along higher trajectories as well; it’s one of the reasons high-ball hurler Chris Young leads the league in homers allowed. So it’s conceivable that pitchers had adjusted to Harper’s power by getting him to chase pitches high in the zone, causing more glancing contact. Yet Harper’s average pitch height in 2016 is only a tenth of an inch higher than in 2015, and his horizontal pitch location hasn’t changed either.

Instead, the culprit for Harper’s problems seems to be genuinely diminished power. Let’s zoom in on a 1.5-foot wide, 6-inch tall rectangle at the center of the strike zone. Hitters normally punish the ball here, hammering balls in that region with an average exit velocity of 90.7 mph — and Harper did even better than that in 2015, hitting batted balls out of that area at 96 mph. (Fourteen of Harper’s 42 homers came from that region.) But in 2016, he’s barely exceeding the league average with an exit velocity of 90.8 mph. So far this season, he’s already made more outs in this central part of the strike zone (57) than he did in all of 2015.

Pitches that Harper was punishing before are now leaving his bat more weakly, and turning into outs more often. Harper’s raw power — once his calling card — has diminished to near league-average levels, and not as a result of bad luck or a new approach from pitchers. Harper’s track record — and the projections informed by it — suggests his slugging will return eventually, but it’s impossible to say whether he needs a simple mechanical tweak or extensive time to recover from a hidden injury.

Even without 450-foot home runs, this diminished version of Bryce Harper remains a good ballplayer, a testament to the breadth and depth of his skills. But at the same time, this just isn’t the Harper we were all expecting to see after the historic performance he produced last season.

Check out our latest MLB predictions.