I don't think the Pacers are interested in trading Ian Mahinmi unless they feel Plumlee is capable of being almost as good as a backup for less money. Although Larry Bird seems to love him and is reported to be the reason that Indy drafted Plumlee in what was perceived to be a reach, perhaps they would be more willing to deal Plumlee. It might be a stretch to think the cost-conscious Pacers are looking to deal a player on a rookie contract.
Courtney Lee is from Indianapolis and fellow local product George Hill tried to recruit Lee as a free agent. He's also considered to be a good fit for their system.
Meanwhile, the Pacers would like to be freed from the mistake that is Gerald Green's contract. Finding someone to take Green off their hands is the only reason I think a trade might be doable.
The Celtics would get rid of one of their excess shooting guards, get back a player on a shorter and small contract, and pick up a young "asset" in Plumlee. I'd argue they should also try to pick up a future second-round pick as part of the deal. I could also see including Fab Melo if Ainge can get back more draft considerations.
Arguably, the Celtics could get more for Lee elsewhere, but the Pacers are one of the teams that would be interested in a backup shooting guard who can shoot threes and Plumlee is one of the few third-string centers who might be an interesting trade target (I don't expect any team to be willing to do a trade for Lee that leaves a gaping hole in their backup center spot unless there are salary cap considerations).