Powe's knee injury history actually has no bearing on the prognosis for recovery for this injury - just looked it up on the Mayo Clinics website:
"The grafted ligament eventually becomes as strong as or even stronger than the original ACL. Graft failure, at worst, is less than 5 percent."
What will impact him is the fact that he may not be ready until January and he'll have missed the entire off-season in terms of working on his game - he'll be strictly in rehab until at least October, which is the start of TC.
Whether he comes back to the Celtics or not will be determined by a number of factors including his rehab, his cost to re-sign, and the team's usage of their roster spots - fortunately for Powe, the team is likely to replace Mikki Moore with someone better, so Powe's spot isn't necissarily up for grabs.
My gut tells me that Ainge will offer him a team-friendly deal that will represent substantial value once Powe returns to full strenght - 3 years with 2 club option or 2 with 1 - something like that. The market isn't going to yield more than 1.5 per i'd imagine, so the team could do a lot worse than signing one of the most statistically efficient players on its roster considering the probability for recovery is high.
They'd be investing in his rehab time, not his chance of recovering.