I don't think it's time to panic about a lack of a backup center. Training camp doesn't start for another 2+ months. Most teams are at or near roster capacity, so it's unlikely that all of the FA options will be gone.
At the same time, the four or five free agent options aren't completely interchangeable. Brad presumably can't wait for months and still guarantee that he will get his favorite guy.
But, I do feel strongly that it would be unacceptable to go into the regular season with obvious holes in the roster. We're now a contender; the "figure it out as we go along" strategy doesn't make a lot of sense at this point. To make a parallel, I was reading an article from Tomase regarding the Sox today, with his premise being that the problems the Sox are having now were caused in the off-season, when management got cute and didn't adequately fill roster holes. That's causing the Sox to lose games, games that will be important if they want to win the wild card.
I'm okay with the Sox not using the TPE. If there plan is to keep the budget at about $20 million over the tax, with two $6-ish million TPEs available if necessary around the deadline, I'm good with that. But, what the team shouldn't do is fill out the remaining roster spots with G-League scrubs, like we've seen before. It's imperative that Brad adds NBA-quality players to fill at least two of the remaining slots.
Not sure what problems the Red Sox could have solved in the off season. Their problem is injuries to Sale, Eovaldi, Wacha, Hill, and Whitlock. That is 5 starting pitchers. I guess you could say first base isn't entirely resolved. Bullpens are never fully resolved. But the Sox were doing just fine, even without Sale, until very recently. And they have James Paxton coming back at some point.
As to the Celtics, your first statement was it is not time to panic over back up center. Exactly, and it could be argued that whoever we bring in (based on the names available at this point) may be the 3rd or 4th center behind RWill, Horford, and maybe even Gallinari. Or looked at another way, the 5th big is what at most will be a 6 big rotation (Kornet being 6th).
I suspect we will mostly play 2-bigs, one more PF'ish, one more Center'ish. But positional designations are very blurred these days. If needed and against some match ups, we can play 2 PF'ish players (Gallinari and Horford) or one PF'ish and one big wing. Any of the above are fine for games here and there in the regular season or for some minutes in individual games. If along the way, someone gets hurt more long term, then you need to do something. That is the "figure it out" part. You don't know how new players are going to fit in exactly, you don't know how young players will develop, you don't know who is going to get injured. That key need that needs to be addressed at the trade deadline may or may not be a big.
This has been a great off season. We brought in size that can shoot in Gallinari (something they have been talking about for a while), we brought in a very versatile and skilled guard/wing type in Brogdon, we brought back all of the core rotation from last season, and we cleared out a lot of incidental players that were taking up roster spots. Now we have a few of the incidental roster spots to fill back up. I understand we don't want to just waste these spots but these moves are not going to impact the season all that much.