(The following is only applicable if they don't finish the season well, where "well" = aren't in the play-in games and play competively against Milwaukee, or beat Atlanta/NYK.)
If they keep playing like this, I don't see how you bring Brad back. Other teams have had injuries, and you don't see this kind of rolling over. People can blame the back of the bench, but most of those guys can play defense, so you should at least be able to grit out some games.
The Celtics stars have been insanely inconsistent mentally and just seem to go missing on both ends for long periods.
At a certain point, it's nice to think your coach is "good", but 90% of the job as the NBA coach of a perennial playoff team is to get them to try hard. Brad has failed miserably at that so far this year, and I don't think it's really debatable.
I know the follow-up question is "well, who could they get who would be better?"
Answer: literally anyone who the players listen to more, which is a pretty f-ing low bar at this point, and one that Stevens has failed to hit 2 of the last 3 years (pending this season's finish).