He's definitely in a slump and it could certainly be because of his conditioning.
His free throw palm grab is screwing up his free throw percentage too. I wish the coaching staff would ban him from doing the palm hold before he shoots, it's a terrible way to build rhythm going into a shot.
Sully has always taken a lot of jumpshots deep in the shotclock which are contested or rushed etc...which probably hurts his overall %.
Still, he's not shooting it well enough and needs to improve long term.
Keep in mind that Marcus Smart has been shooting 33% from the field which is something I'd be more worried about right now, especially his 23% from 3 point land.