One thing I think is getting lost in the discussion is that there are two types of unplayable players:
1) The ones that everyone knew were unplayable when the roster was made, but they're on it for development, contractual reasons, or because you simply need bodies/expiring contracts for trades. (i.e. Pre-season unplayables)
2) The ones you thought would be playable when you made the roster but were wrong about. This includes season-ending injuries. (i.e. post-season or hindsight unplayables).
By the end of last season, it was clear the Celtics had 6 unplayables -- 2 were developmental (Hauser and Nesmith), and 4 were due to a need of bodies (Stauskas, Fitts, Morgan, and Kornet). This was because the team started the season with a ton of Type 1 unplayables (Fernando, Parker, Hernangomez, Nesmith, Langford) and chose to spend the trade deadline (wisely in my opinion) upgrading the rotation instead of fixing the depth. It's not clear they could have done both, given how weak the buyout market was and how few low-contract rotation players were moved around the deadline. The only two players that contributed in the postseason from the buyout market were Goran Dragic and Jevon Carter, and so the Celtics were left to fill out the roster with fodder.
Of the current 12 players under contract with the Celtics, only one player meets the pre-season definition of unplayable, and that's Kornet. The Celtics clearly think Hauser will be playable, which is why they did the contract gymnastics of signing him to a 3-year deal with 2 years guaranteed, rather than just letting him stay on his non-guaranteed deal this season before hitting restricted free agency. That shows they think he'd get a decent offer next summer they wouldn't want to match, and that's only happening if he's in actuality playable this season. It remains to be seen if their player evaluation is correct, but if he's unplayable, that's a fault of evaluation and not roster construction.
In a perfect world they'd have 13 playables and 2 developmental unplayables when they break camp, but it's all right if it's 12/3 or 12/2, because finding one more player during the year is not a tall order when the rotation doesn't need the upgrading it did last winter. 11/4 or 11/3 would be less good, but still fixable, although I don't think that's where this ends up anyway.