What's wrong with Giancarlo Stanton? It's the question of the moment in Miami -- and really across baseball -- as the sport's preeminent slugger has fallen into the deepest slump of his career.
Over the past 30 days, of the 228 hitters with at least 70 plate appearances, only one (Boston's Christian Vazquez) has a line weaker than Stanton's .118/.211/.206. Worse, Stanton's strikeout rate of 46.1 percent in that span isn't just below average -- it's more than twice the Major League average of 21.1 percent. In baseball history, no player has had a season of at least 300 plate appearances and put up a 40 percent strikeout rate. Elite power or not, no hitter can survive with that extreme lack of contact.
Unsurprisingly, there's no shortage of theories as to what's happening. One popular narrative holds that Stanton, wary of being hit by the ball after being beaned in the face by Mike Fiers in 2014, is now standing further from the plate and can no longer reach outside breaking pitches. But the idea that a pitch from two years ago is suddenly impacting him now doesn't hold water -- after all, he had a vintage Stanton season in '15 and he was off to the best start of his career for the first month of '16. Why would that suddenly bother him again in the first week of May?
Remember, the first week of May is when all of this really started. On May 6, Stanton crushed a 475-foot blast off of Hector Neris, measured by Statcast™ as the longest home run of the season until Nomar Mazara hit a 491-foot blast on May 25. At the time, he was hitting .274/.391/.632, a line that would be his best over a full year. The next night, Stanton went 0-for-4, but he struck out only once, and even one of his outs was crushed, a 112-mph grounder to shortstop Freddy Galvis.
If there's really a turning point to Stanton's season, it was the following day, May 8. Despite being off to a career-best start and the team having had two days off in the previous 10 days, he sat, striking out as a pinch-hitter in the ninth. Stanton struck out three times the next day against Milwaukee, and he hasn't been the same since. Just take a look at his 20-game rolling strikeout percentage and see if you can see where things turned:

For a player with such a long, clear record of excellence, it doesn't stand to reason that Stanton simply forgot how to hit, and a video comparison of games from his slump against games from 2014, '15 and his hot start to '16 doesn't show much in the way of a changed stance or position in the box.
That being the case, the most likely cause would seem to be that Stanton is not fully healthy, and that's not totally a specious assumption. Remember, he sat out seven straight games in late May with "right side soreness," an issue that had apparently first popped up in Spring Training. (He's since sat out three additional games in June.) As recently as Tuesday, Marlins manager Don Mattingly was admitting that the side was still an issue, telling MLB.com's Joe Frisaro that "the sore right side that caused [Stanton] to miss the final week of May has impacted his swing a bit."
"Side soreness," while an admittedly vague description -- it's also been described as a "sore torso," which doesn't help much more -- certainly seems like a problem for an athlete where twisting at maximum effort is a huge part of his job description. After all, while there is some truth to the fact that Stanton's inability to hold up on low-and-away sliders is impacting him (he's hit just .211 on the pitch in his career, with his highest strikeout rate), it's not like that is a new condition. Look at Stanton's batting average per pitch on all pitches, splitting between 2015 through May 7 and since May 8:

Stanton is having a hard time making contact on pitches in the zone, particularly those up and in. If he'd actually moved off the plate, he'd in theory be in a better position to hit those, but that's not what's happened. In fact, since Stanton's mechanics starting his swing look the same, a reasonable assertion would be that if the side was an issue, not only would he struggle to make contact, but he'd have a hard time being on time with his swing. That's exactly what's happened. Over the past few years, Stanton has always pulled between 42 and 47 percent of his batted balls. Through May 7, he pulled 45 percent this year. Since May 8? Only 38 percent. 
Whatever's wrong with Stanton, it's still wrong. Players this elite and this talented don't simply break overnight without reason, and we know that the side bothered Stanton in the spring and in late May, and it may still be an issue right now. We've seen players struggle while trying to play through a problem before, and with Christian Yelich, Marcell Ozuna and Ichiro Suzuki all having very good seasons, the Marlins can spot for Stanton for a few weeks if they choose to give him time off. Whether it's mental, physical, or both, what's happening now isn't helping anyone.