Knowing that Hudson is up there in age for a rookie, maybe the Celtics drafted him as the most NBA-ready player available, and they'll only keep him if he proves he can play right now. This hasn't been something Danny has done in the past with his 2nd round picks, but maybe something about the team's current situation caused him to think that way. Perhaps if Lester doesn't show that he can play, they would cut him and keep Sweetney just to have an extra body around.
Yes.
Plus, I think people are overstating the importance of the position that the #15 guy on the roster plays. #15 isn't a player that you are even thinking about as getting on the court. If it's a power forward, or a center, a PG or a wing, it's almost irrelevant. #15 is usually a guy that's either left over and useless, or a project that you're hoping might be of assistance in a year or two (when the rosters' composition could be entirely different).
If Sweetney can play, they'll bring him aboard, stash him, let him keep working himself into shape, and then maybe that'll let them make a BBD or Scal trade easier mid-season. We're likely to trade one of those guys at some point, so the fact that the frontcourt rotation is pretty well filled-up today doesn't really change how you think about that last roster spot.
I do think it's up to Hudson, if he shows that he's real close to being able to contribute, they'll keep him aboard, but if they see that he just can't match up there isn't much point keeping him around.
And, if they're both about the same in terms of ability to contribute... it's easier to find a smallish wing man in the D-League than it is to find a big.