Bradley is the only Celtic that really makes sense to replace Middleton.
I wouldn't be in a rush, but I could see:
Bradley, RJ, Young
for
Henson, Maker, 1st rounder
Sheds a roster spot and brings back 13 years of control over the incoming players (including the pick once drafted). Henson might grow under Horford, or isn't a bad value under the current cap on his deal. Maker can go to Maine, further simplifying the roster.
Of course, it also leaves us with the same problem Milwaukee is trying to solve- no SGs who can shoot.