I don't think the Celts have the size or length on the wing, or the athleticism, strength, and quickness in the interior, to really be a great defensive team.
Stevens will cobble this unit into an above average defense again, though, I have no doubt.
The offense will be the bigger issue.
Still, I think we can expect to see a number of games where the Celts get pounded inside.
The nice thing about the modern NBA, and probably part of the reason why we haven't emphasized getting a rim protector, is that there's only a handful of teams that can consistently pound the ball inside. Defending the perimeter, especially the 3, seems more useful across the board these days. I think we were top 5 in opponent 3 pt% last season (can't look it up), so hopefully we can keep it up.
Well, I don't know, I think "pound inside" has just changed in how it looks. Once upon a time, defending the paint meant having big post defenders who could give guys like Dwight and Bynum trouble (e.g. our friend Perk).
Now, pounding inside is all about attacking the rim out of the pick and roll and on backdoor cuts off perimeter ball movement.
I think the Celts lack the length on the perimeter to make it really hard for opponents to get passes inside, and they lack big men with the lateral quickness, explosiveness, and length to swallow pick and rolls and discourage shots at the rim.
So I don't think the Celts will be bad defensively, like I said, but I expect them to get exposed inside fairly often, while making the most of the talent they have, especially defending the perimeter.