Simmons was more efficient the year before (better eFG%, TS%, ORtg) when his usage rate was 2% lower (3% in conference play). Some of that seems to be TOs, especially in conference play, and another big part is 3pt%, which could be usage related but could also be random.
Also, while Simmons is playing in a better conference, he's also going to playing with significantly better offensive players. I haven't watched much Ohio basketball, but I don't think he was surrounded by guys like Wagner, Robinson, MAAR, etc., or for a coach like JB. This does include some unknowns -- how good will Matthews be? The freshmen?
I think JB will be able to limit Simmons's TOs. I wouldn't be surprised to see him with a usage rate around 23-24% with good efficiency. I don't expect that he'll use possessions as much or efficiently as late season Walton.