I don't think he is completely done. He's not that old and plenty of people come back from torn ACLs. He may not be 100% next year, but he still has basketball talent. His status as a knuckle-head will probably hurt his asking price more than the injury per se.
I, for one, would love to see him work his way back next year on the C's bench. Playing as a reserve would be perfect for getting back into form because he can play against second-stringers while limiting wear on his knee. He and Quis are good friends from their days in Dallas, so maybe he considers Boston an option if Quis comes back. It's nice to think about a bench consisting of Robinson, Quis, and Howard.