With regard to Southgate not playing him, that's a combination of not knowing what to do with him, and an unease with what he does - or more specifically doesn't do - off the ball.
I think United is a decent move for him. He needs to be somewhere he'll be pushed constantly, somewhere that will bench him when he's not producing. Dortmund has been great for him up to a point, but it became a situation where even a casual performance from him looked more appealing to the manager than what he had to replace him with.