The knee tendonitis has definitely been slowing him. He's been trying to take it inside constantly but doesn't have his same burst and lift and isn't getting the calls he used to. He's still been remarkably effective though considering. Nene is just not going to make many bad plays. It's probably not that uncommon for him to play a 30 minute game and make 0 mistakes.
I wouldn't be in a rush to deal Nene, but I would listen to offers. He could help every team in the league pretty much because he takes nothing away from the table on the court and his only "flaw" is his health.
If he could get me back a big man with long term value, I'd consider making a move.