Yeah, his experience in OKC shows that you don't want to give him more than spot minutes at PF, and I'm not convinced you want to play Pierce all that much at the SG. Both guys should be playing the small forward.
The bigger challenge/issue is his contract status. He's a restricted free agent and pretty likely to command a multi-year offer; based on his production as a starter, figure minimum of three years, probably something between the MLE and $10M/year. If I were guessing, I'd say he's likely to sign an offer sheet for something around the deal that Perk got; four years, ~$8M per.
Celtics have ~$28M in salaries committed after 2012, just counting Pierce, Rondo, and Avery Bradley's $1.6M. Add in $8M for Jeff Green, we're up to $36M. You've got cap holds for players like KG and Ray that are expiring, and you've got small cap holds for the other roster spots.
Even if you renounced Bird rights to KG and Ray, that's not leaving much room to acquire Dwight Howard or any other max-level free agent that summer. If you commit to Jeff Green as one of the team's building blocks, that severely limits what Danny may be able to do in a years' time.
I haven't see enough from Green to make me want him to be one of the building blocks, especially not when Pierce is already signed through '13-'14. And, it's hard to see Green wanting to commit to staying here, when he *is* good enough to start for a lot of teams.
Just not a great fit in Boston.