Celtics are at 13 players, they have two weeks to get up to the minimum of 14.
They could still use the TPE generated by Springer up until the deadline, but I'd guess this is just about the tax savings and they'll probably fill out the roster with a minimum signing.
Almost 2/3 of the way through the season, top 9 rotation players are all healthy, second in the east on pace for 58 wins. The C's are in a pretty good place, there's not a whole lot they can reasonably do to upgrade their roster in a way that will actually matter.