Dylan Hernandez in the Los Angeles Times speculates on the causes of slump of Mookie Betts. He suggests the relatively small player is breaking down:

“I think that’s fair,” Roberts said. “Mookie’s never going to use ailments or things as an excuse. He’s a guy that prides himself on playing every day. But when you talk about the swing and performing consistently, you talk about the back, you talk about the front shoulder.”

LATimes.com

I am on board with the injury theory. Any number of players go through season long slumps only to reveal at the end there was a nagging injury. At this point, if a player is in a long slump, I assume an unreported medical problem.

The other (not necessarily competing) hypothesis not discussed is the ball. Size does matter when it comes to power. Betts needs to work harder to generate power than say someone tall and wide like Aaron Judge. If the league knocks some carry off the ball, some of those home runs come down in a glove. Some of the deep doubles in the gap get run down. His percent of fly balls going for home runs is down. Combine a bad left shoulder with a dead ball, and you have a recipe for disaster.

Betts’s big boost in offense came from a change in launch angle to generate more power. Maybe, until he’s healed, Betts needs to return to his roots.