There's rumors that Atlanta, being the only other team besides Boston right now to have a meeting with Dwight, wants to pair Horford and Howard together. Howard is from the Atlanta area and has long wanted to go back and play there, so I would figure that they would have the lead over us in the Dwight sweepstakes.
However, Atlanta doesn't necessarily have the room straight up for Howard AND Horford with Horford and Bazemore's impending free agency and cap holds. Now, they could possibly make room for Howard without moving Milsap, but that would really further decimate their roster and it's still not clear that they'd be able to sign Howard in cap space and keep BOTH Horford and Bazemore. There's an obvious fit issue with the trio of Milsap, Horford, and Howard, too, so if they want to make room for Howard and keep Baze and Horford, moving Milsap for non-guaranteed salaries makes the most sense.
In comes the Celtics. We only have a day or so before we have to make the decisions on Amir and JJ's non-guaranteed contracts, so we'd have to do this quickly. However, a Milsap for AJ, JJ, and the Clippers pick we just got would work well for both sides. Atlanta clears the cap space to pair Howard with Horford while retaining Baze, and we get a very suitable two-way star to try and convince Durant to sign here. (Obviously you'd try and see what Durant thinks about this move via backchannels first)
We'd then just need to try and trade for a cheap defensive big, like Noel, and we'd have a very good two-way roster that is on par with Cleveland as a title-contender in the East. You'd then end up with this roster (assuming you pretty much just traded Brown straight up for Noel - which makes sense for both sides since they'd probably not get Barnes in this scenario and Brown plays a position of need in the wing):
PG: IT, Rozier
SG: Bradley, Smart
SF: Durant, Crowder
PF: Milsap, Mickey
C: Noel, KO
The only concern would be Milsap's contract status, because he'll almost assuredly opt-out next summer with his player option. However, that might also be a good thing for us since next year's free agent crop is absolutely loaded with talent.
So who says no?