This was asked in yesterday’s game thread and it bears further discussion. Maybe a way too simplified answer is that blister concerns are there and the Sox have instructed him not to use it so much. I also think when he starts to miss with the curve early on, he quickly loses any confidence and goes back to the fastball, and here and there a changeup.
It’s so damn frustrating because we know he has the talent, but relying on two pitches to get him through most AL lineups will get him shelled more often than not.