Most teams do not have 4 fringe players on their bench and very few even have 3.
I will just pick one for example, Golden State. The end of their bench was Juan Toscano-Anderson, Damion Lee, Moses Moody, Andre Iguodala, and
Jonathan Kuminga. So 3 fringe NBA players, one near spent vet, and a promising but not ready for NBA rookie.
Moody, like Kuminga, was a lottery pick. Toscano-Anderson just finished his 3rd year and has started 28 games in his career averaging over 17 mpg. He signed a contract with the Lakers in the off season. He is no where near a fringe NBA player. Lee just finished his 5th year and has been a regular rotation player, including 53 starts, averaging 21 mpg in his career. He isn't a fringe NBA player and both he and Toscano-Anderson basically played in every game they were healthy for the Warriors. They weren't just the occasional player. Lee signed with the Suns, which will be his 3rd team. Iggy is obviously a spent vet, who they brought in for leadership. He was a fringe NBA player last year, but he was the only 1 on the Warriors roster.
If Gallinari didn't get hurt, we would have had probably the best 1-10 in the league. All solid NBA proven players. Then we have Vonleh (probably), Hauser, and Kornet, all players with NBA experience. These guys are just fine for 13-15. That of course leaves slots 11 and 12 (plus or minus). I agree, it would be nice to fill these two slots with an established vet wing and an established vet center. I believe Vonleh may actually be fine in the role of a 11th or 12th man big so that may be covered (then we can find another scrub for the 14th slot). And Hauser may surprise, I am not expecting that but he has had flashes.
I just disagree that guys like Vonleh that have been out of the league for years and weren't very good when they played are NBA caliber players. Maybe he surprises, and it is fine to have one or maybe 2 of those guys on the roster, but Boston will have 5 of them. The Gallo injury aside, the back end of the roster has been terribly managed. It was unacceptable for Boston to not use the TPE with that many open spots on the roster. To not be willing to bring in actual NBA players, also unacceptable. Boston is treating the bench like they are rebuilding team, not a team actually allegedly trying to win a championship. I also disagree with the idea that Boston had the best 1-10 in the league pre-Gallo. There are several teams that are better 1-10 and all of them have better players 11-15 (on the whole). As an example, Golden State's top 10 players this year are: Curry, Klay, Wiggins, Dray, Wiseman, Poole, DiVincenzo, Kuminga, Green, Looney, and #11 is Moody who is a 2nd year lottery pick that just turned 20 and started 11 games last year. They have a rookie 1st round pick after that. That is a better deeper team than Boston, but they do have some open roster spots and like Boston should use them. The main competition in the East, i.e. the Bucks, are also deeper 1 to 10 and have a better back end of the roster. Boston's real advantage on teams is the 4-7 spots. Very few teams have the quality Boston does there i.e. Smart, Rob, Al, and White, but most of the real competition are better collectively 1-3 and 8-15. The 1-3 is hard to fix, the 8-15 is not. That is the issue.
The problem is that it takes two to tango as they say. Everyone seems to make the assumptions that these FAs or trades are right there on the table and it is only a question of the Celtics pulling the trigger. That they are being cheap or maybe just demonstrating poor judgement or something. Just sign Dwight Howard. Just use a TPE and trade for vet wing xxxx. I don't see it that way. I believe they will do something but they are waiting as Dwight Howard doesn't want to play in Boston and no team is offering a decent vet wing for peanuts right now. I think many of the remaining viable FAs are waiting too.
I am not worried or complaining about this roster depth. I think they are fine. Losing Gallinari is an issue and may require some change of plan but give it some time. No need to be rash here. We are fine and will likely make some moves along the way to get even better.
Boston has 2 centers on the roster and both of them will probably start. Boston has 2 SF's that you could actually play meaningful minutes too, and like the centers, they both will likely start. There isn't a pure PG on the team either. A bunch of combo or scoring PG's, not a guy that is a pure PG. Boston, even before Gallo went down, had significant holes on the roster.