Humphries is just going to be difficult to move unless the Celtics throw in a first round pick, which would be monumentally stupid. As much as Ainge seems to like low basketball IQ players, he's not that stupid.
There aren't many trade exceptions out there, if any, that can simply absorb Humphries' salary. That avenue seems unlikely. Contending teams or teams battling for the playoffs tend not to have 12 million in cap room, so they'd have to send out a big chunk of salary to make a trade work. Humphries isn't good enough to merit two or more role players, nevermind if Humphries puts up solid numbers on a bad Celtics team that he'll have only put up solid numbers on terrible teams.
A team trading for Humphries will want to unload a contract with more years on it than Humphries'. Something like Humphries for Marcus Thornton who has 8 million left after this season, that kind of deal. But I doubt Ainge would be interested. Humphries' salary help facilitate the Nets' trade but its size seems like a big impediment to unload.