If the Celtics bring back Tony Allen and Nate Robinson, the combination of those two plus Avery Bradley should be plenty good to back up Ray Allen.
If the Celtics can trade Sheed's contract, I think the target should be:
1) Fits within the defensive system
2) Is at least 6'7"
3) Is best suited to play small forward behind Pierce.
Pierce has a four-year deal and Bradley should be looked at right now as the shooting guard of the future, so I think that looking for a young back-up wing with potential is the wrong strategy. The goal should be to find a serviceable back-up small forward, someone who is at his peak, and maybe also a player suited to be the fifth big.
My ideal would be a big 3 with range who in theory could play the 4, but lacks bulk, and probably should only play the 4 as part of a smallball lineup or against weaker, reserve 4s. I'm not sure what players fit this mold. I guess I'm looking for something like a poor man's Mike Dunleavy.