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.
Hayward might not have been the lynchpin, but him leaving had more a nuanced ripple effect on the team's defense in the sense that it placed a lot more offensive burden on the Jays, which was only enhanced by Walker being out of the lineup to start the season. Expecting those two guys to pretty much be everything for the team offensively and still be able to play at a high level on the other end is a tall order.
Funny enough you mention the bubble, you could see the wear and tear getting to the team towards the end of the playoffs last year.
Hayward leaving definitely placed more of a scoring burden on the Jays. That's a good point. Also Kemba's knee issues exacerbated that. You also have the fact that Hayward's absence means that you have to rely more on Semi / Grant / Nesmith for backup wing minutes. That also resulted in the use of the awful two-big lineups which were so bad.
On top of that, Wanamaker leaving and being replaced by Teague / Pritchard meant you had a solid though not great veteran player replaced by a washed up vet who has never really been good on defense and a rookie whose strengths do not lie on the defensive end. That didn't help either.
I'd guess more than that was the lack of continuity. A lot of team defense is based on knowing the tendencies of your teammates and where they are going to be. Boston had some new faces, but more importantly couldn't practice and had a lot of guys miss a lot of games all season long. There was just no continuity on the team which really hurts a team's defense. I mean Tatum, Pritchard, and G. Williams were the only guys to play in at least 60 games, last year 6 guys played in at least 60 including some of the better defenders in Tatum, Theis, and Smart. Also, no one in the playoff rotation last year played less than Hayward's 52 games, but this year Smart, Walker, and Nesmith were all in the 40's. Plus you had the trades this year that didn't exist last year. I'd guess next year if Boston has more continuity, less missed games, etc. that the defense will just naturally revert back to some of what it was last year and that this year will be thought of more as an outlier.