Middleton has been ruled out for tonight's game. It was a clear ankle sprain and he definitely needed some time off to heal, but this is now approaching 7 weeks.
Middleton's problem isn't so much that he gets injured. Any player would have been injured by Durant undercutting him like he did. It's that instead of 3-4 weeks to recover, it takes him 2+ months to recover from any injury he gets.
I can see him coming back, getting back up to game speed, and then straining a hamstring in round one of the playoffs and having to sit then.
Because of the way the salary cap works, they had no real choice other than to resign him, but you can't count on him at all at this point.