It isn't a hard and fast thing but losing him for nothing, losing him for an Okogie clone, losing him for 2 Damion Lees, losing him for an expired contract etc. seals our fate IMO. If we can get another player that you wouldn't be embarrassed that your team drafted him with a very high pick, then that's enough value. If you can trade him for 2 players that you'd want in your 7 man rotation, then that's enough value.
But one way or another, we need good value out of him and with his contract I think it's more likely that he can give us that value on the court than some other team will trade for him. Right now, he has less value than his contract and the new CBA rules make him a much less desirable trade target than he would have been just a few months ago. JMO.