To be honest, at the moment, I just don't think he has the mentality to play for Manchester United. He seems like a player who is nervous to be playing at Old Trafford in a United shirt, and in-turn, doesn't really look confident in anything he does.
When he's got time and space, he's fine... in the second half, Bayern (bizarrely) let him just bring down DDG's long punts, which is his natural game and something he could probably do in his sleep - fine... but when he's put under a bit of pressure, or he's challenged by the opposition, that little bit of panic seems to set in, and he just doesn't look strong, composed or confident in the slightest. This was exemplified by the occassions where he seemed to refrain from challenging for balls in the air - or just not challenging strongly enough in general. There was a cross in the second half from our left hand side that went to the back-post, where Fellaini was lurking, and - even though he had a bit of a run on I think Alaba - he didn't put in any meaningful challenge whatsoever to try win that header and score a goal.
Counter this with Vidic, who has such a maniacal desire to win the ball, and confidence in his ability to do so - that he rarely loses a header in the air.
I remember watching Fellaini come to Old Trafford with Everton and bully our players up and down the pitch... that bully is nowhere to be found at the moment.