A year ago, this was a different question. Now, no one know's if Favors will ever be the player he was. He has chronic lingering knee issues. Now, Bradley is the more valuable player.
Bradley gets injured a fair amount too, but its important to not that nothing that has happened to Bradley so far should affect his long term athleticism. That's definitely not true of Favors.
Ainge loves Bradley, and rightfully so. No trade!