I think our roster is just fundamentally flawed in composition. Even if we were healthier, I think we would've have been underwhelming based on the high preseason predictions that were thrown around here before.
I agree that the bench needs more vets. But re: the defense, what is so different about the roster composition between last season and this one? The big change was losing Hayward. Hayward was a good defender; was he really the lynchpin of the Celts' elite defense? I'm skeptical about that. Especially since they remained very good on that end in the playoffs without Gordon.
At least on paper, exchanging Wanamaker and Kanter for Teague and Thompson doesn't seem like it ought to have hurt the defense, either.
Defense even more than offense is a team effort. I do think there is something to the idea that the more young players you add into that equation the harder it is to keep the whole thing from falling apart. The Celtics probably do have more than a critical mass of unseasoned / rookie contract players getting minutes, especially in this season when players were constantly missing time.
I expect some significant changes in the middle part of the roster as well as the back end. I will be surprised if Kemba, Smart, and Thompson are still on the team next season. But even if all of those guys return, I think they can get back to elite defense if they have better health and they add a couple of disciplined vets that bring toughness and energy on the defensive end. The Celts really miss that kind of energy they used to get from guys like Jae Crowder / Amir Johnson / Marcus Morris / Aron Baynes etc.