What worries me about Jimmy Butler is fit.
If he didn't fit well with Derrick Rose, what would make him fit with Isaiah Thomas, who in a way, are similar players (ball dominant PG's who likes to shoot)?
Fitting with Derrick Rose doesn't matter because you can't have a good fit with garbage.
If you are worried about fit, you should consider what kind of offensive system Fred Hoiberg wants to run, how similar that is to how Brad Stevens wishes to operate, and how Jimmy Butler has been adapting to that system. If he wants to be "the man" and create shots off the dribble, that is a bad fit. If he is a willing passer who will embrace playing off the ball within the offense, that is a good fit.
Is Butler the sort of player who is going to pout if he is put in a situation that causes his PPG to go down? If so, should Stevens be willing to abandon his system some of the time and let Butler play iso-ball to keep him happy?