This move would send us backwards temporarily (this season), but I think forwards overall and I would do this move if I'm the Celtics.
That said, there is no way the Nuggets do this trade. First, how on earth does getting IT help the Nuggets franchise? Second, IT's window does not overlap with Denver's, it would regress the development of Mudiay (who I'm skeptical on anyways), and they'd lose lots of their best assets. Stupid stupid deal for them to accept.