I say Stevens, though Ainge has made his share of mistakes. The League has basically passed him by: it is no longer about the draft. It is about amassing free agents who want to play together, and filling out the team on the cheap with veteran role players who want to win. The very formula Ainge used to win a Championship in '08. There's the irony. But the ownership cut off the money, Perk got traded and KG got hurt. So Danny went the draft route, and after 13 years, he has nothing to show for it --by Celtics standards.
A tip off on the problem was Ainge's inability to trade for Vucevic instead of Fournier--another ho-hum 3P shooter--when everyone in the league knows the Celtics weakness is the lack of a rim protector. Paint defense. Not that Vucevic is Bill Russell, but he makes up for it other ways. Rob Williams? He's been brought along so slowly he hasn't had nearly the effect he should have had. That's where Stevens comes in. The guy can't coach bigs. Doesn't want them, never had them at Butler ("we could never recruit any good ones, so we went to a small ball game.") Well, small ball is not the way Celtics teams win championships. Or any team, for that matter. Ainge knows that, and should have realized it when he hired him. Sure they had a good run with Al Horford, who fit as a 'tweener big. But they haven't done much since then. Both Ainges know the problem--Austin signed Tacko who? Danny signed Rob Williams--too young, not durable. And Thompson hasn't done much. He's passed up many opportunities to sign leading bigs who could make a difference--guys like Drummond, Cousins etc. I suspect Stevens has a personnel say (one-third, as Ainge was quoted), and that has been a fly in the ointment.
Time for a change. The current team is going nowhere, by Celtics old standards. Danny has never really built a balanced team around Tatum and Brown. Just tried to copy them with lesser guys. And then he lost a few stars to bigger markets--maybe because of his coach. He hasn't done a very good job lately molding a balanced team with trades and free agents. Rozier, for example, a guy he let go, turned out to be better than Walker, the expensive free agent he signed without taking a good look at his knees.