Two things
There are a lot of good PF free agents this year on the market. Sheed, Boozer "most likely", Odom, Joe Smith, Anderson Varajo "player option", Chris Anderson, Antonio Mcdyess, Chris Wilcox, Stro Swift, Marion, Drew Gooden. All of these guys are unrestricted free agents.
The economy is in the tank, the salary cap is going down next year and a more teams will be playing the luxury tax. After reviewing the list above and who will end up where. No one is paying baby more than 3yrs $12mm. My best guess is that he shops around and stays with the Celtics for 3yrs $10mm or 2yrs $7mm. He might take a small paycut to stay with this team.