Yeah, a red is harsh, but I can see why the referee gave it.
Madrid were robbed of a clear goal (Higuain's) in the first half and of a penalty (Rafael's handball denying a goal) before the red card incident so I can't see how Manchester United can complain. It should have been 2-1 for Madrid before the red card anyway.
It was just poor referring the whole evening, no clear agenda on the referee's part.