I'm scratching my head over why people are so sure he's gone. If he's set on leaving, there is no reason to wait to announce. If he wants to stay, it makes sense to wait to announce so he can get the last workouts in and form relationships with team management for next year.
To me, the line that "the decision has already been made" makes him much MORE likely to stay, not the other way around. It's the only thing that makes sense. Guys who've made up their mind to go have declared publicly that they're leaving already. There's no reason to wait.