What explains Mookie Betts’ drop-off?

Ten-win seasons are hard to repeat unless your name is Mike Trout. Though Mookie Betts had one of the best seasons in modern baseball history—a .346/.438/.640 triple slash with 10.4 fWAR— in 2018, he has now found himself not even the best player on his own team as Xander Bogaerts has had his own mini-breakout.

It was kind of obvious that he would see some decline given that framework; most projections probably thought of him as a 130 wRC+ hitter more than a 185 one, but 121 as of June 18th is still... just a little low, right?

It seems even lower when you start comparing his major peripherals from last year to this one, and it makes you scratch your head more.

He is swinging and missing less, swinging at the same rate, and is making contact more. By Statcast, the story is incredibly similar. By average exit velocity, he was ranked 17th last year, and 17th this year. He is ninth in the league in Barrels, as opposed to fifth last season.

Barrels and expected statistics seem to show the biggest difference, though, where his xwOBA of .380 trails to last season’s whopping .433. Yet, it’s still higher than his current .362, meaning that a regression could be in order. Yes, even most of Betts’ second half in 2018 was a regression to his traditional xwOBA mean:

This could be because of a slightly declining launch angle, from 22 degrees to 15, or just a smidgen less hard contact, down about eight percentage points from last year.

The big question is how, and simply enough, it has to do with pitcher approach. Betts has a significant platoon split this season, showcasing a 212 wRC+ against lefties and a 176 against righties last season, contrasted by a 59 and 143 this year, respectively.

Why so bad against left-handers? If you look at the heat maps, pitchers traditionally thought that going after him on the outside corner was the smartest idea:

This would generally make sense, if his weakest spot against lefties wasn’t actually inside:

So, pitchers in 2019 have followed that up with an adjustment that would allow them to get in on the hands of Betts:

The result has been Betts being forced to feast on any measly pitch on the outside half:

It’s not the end of the world, obviously. Even with a better left-handed strategy, right-handers still haven’t had much success, and the already built-in exit velocity numbers mean we should see a rebound in the near future. To be perfectly fair and honest, ZiPS projects that he will finish with a 5.6 fWAR, after all.

Yet it’s hard to shake the notion that the ten-win season was a “fluke” in the sense that he will look more like 2019 or 2017 version of himself rather than the 2018 variety. Maybe it doesn’t matter as this could be a fluctuating trend of elite to merely very good. Either way, pitchers from the left side pounding him on the inner third will have to be remedied one way or another, fluke or not.

Either he will need to adjust his stance a little further from the box, or get his hands inside quicker. Something will, and I believe it has to, give. Betts won’t win the MVP this year, and it’s hard to tell if that level of offensive production can be seen again, but his adjustments in the second half will be a clue as to whether it will be.