Ray was a free agent. It's not his responsibility to fill all the team holes for the Celtics.
If the Celtics wanted Ray in green, they should have talked extension with him last season (as he requested), rather than telling him that he was being traded to Memphis.
The Celtics decided to gamble on Ray's future. That was their choice, and a legitimate one. However, by letting Ray hit the open market, they knew this was a potential consequence.
I don't think we can blame all our problems on Ray, though. What exception would we still have if Ray signed here? The Green contract was basically worked out in advance of Ray signing in Miami. We still wouldn't have had any money for a legit center, regardless of what Ray had done.
Regardless, we have Terry and Bradley, both of whom can play SG. We can offer Pietrus a raise to come back. We have sign-and-trade options for Courtney Lee. We'll be fine.