Paul Millsap was supposedly a trade deadline target of the Celtics in 2013 and Jackie MacMullen reported that he was "Plan A" in free agency. I still believe that Ainge would have kept Garnett and (maybe) Pierce if he'd been able to work a deal to bring in Millsap.
Millsap is a hard worker who has shown with Atlanta that he can be a capable team defender, especially now that he doesn't have to compensate for Al Jefferson. He's developed into a credible stretch four who gives good effort on defense.
Although he says he has forgiven Danny Ferry, maybe that incident makes it easier for Millsap to leave Atlanta for big money elsewhere, even the Hawks do well this season. And maybe Atlanta isn't willing to give a near-max contract for a 30-year-old free agent.
I would be willing to give up Wallace's expiring contract, the un-guaranteed final season of Faverani's deal, and a couple of firsts for Millsap. Then, maybe, the Celtics can move Sullinger in a package for a center. A sign-and-trade would make it possible for the Celtics to re-sign Rondo and Green (if he opts out). Or you can send Sullinger to Atlanta.