You need more than 1 play to determine if his rehab didn't go well but in order to throw any kind of ball with any velocity you need to be pushing off the ball of your rear foot...
I've never really looked at his mechanics before but you should also look at his pre-injury throws to see if he is one of these people that don't use their lower body very much when throwing.
He is getting very little push-drive off of that foot.