There's no one who they can sign who will be plugged into the rotation for 10-15 mpg, so I think the best course is to wait and see if there are any injuries that create an obvious position where a signing for depth is needed. I'd also wait and see if the likely first round opponent creates a possible need for a specific skill set. A stout post defender may be a tool to put in Brad Stevens' box if it looks like the Celtics will have to deal with Al Jefferson.
They could sign someone to a non guaranteed deal to help with trades this offseason.
But there's no reason to do that until the last day of the season, in case the guy you sign doesn't play the same position as a player who gets injured in the last week of the season.
Everyone's going to have cap space, so there's likely not going to be a need for an unguaranteed contract to match salaries. With the number of picks and deep roster the Celtics have, it's really not a good situation for a guy who is hoping for a chance to impress in summer league and latch onto a team. A break-glass-in-case-of-emergency veteran seems most appropriate.
I'd even suggest finding an enforcer who is only going to get into a game if a Celtic gets injured. Ivan Johnson just got banned from the Philippine Basketball Association a few weeks ago. I'd strongly consider someone like him.
