I don't count either Milan games, it was the start of the season the Real team did not gel yet, and i am sure if they played after December-January they'd beat Milan easily.
I do think however that United's tactics and fast tempo would be too much for Real. Real despite having a strong midfield and a deadly attacking force are really soft in defense. I don't know if you watch their games, but if you do, i have to point out that they dominate most of their games and their defense is very rarely exposed, in the rare ocassions when they are exposed, their defense is rather poor, Albiol and Garay are not the type of defenders you want playing when you deploy a high defensive line, and sometimes their lack of pace really shows. I think against a counterattacking team like United, Real will find it a bit too hard.