Mihm is the best available center. He is a somewhat slow but effective half-court low post guy who can board, defend, and block shots.
He is a restricted free agent whom the Celtics may not want to resign due to committments to Blount, LaFrentz, Kendrick Perkins, and a focus on the running game and transition basketball, which is not something suited to Mihm's game.
Might Pollard benefit from a more full-court attack? I'm not sure, but he would seem to be a better fit in Boston than Mihm, and Mihm a better fit in Indiana than Boston.
Boston doesn't want to play Mihm 5 million, so they probably would not want to have to pay the older Pollard 5 million, so you would have to add in at least a future 1st rounder to entice them.
I'd do that. Maybe the Celtics would too, if they don't think that Scot is washed up.