His knees (in all likelihood) have a degenerative condition. Its not just a matter of pain or caution, its a problem that he will continue to face for the rest of his career (life).
The real question is what can be done to slow down the degeneration, if anything at all. That is the question the 76ers will have to weigh before resigning him
With that being said, do you still see him as a viable long term investment?