Whenever United think they might not be technically superior to their opposition, they compensate with energy, pressing and harrying, winning most of the 50-50 balls, forcing mistakes from the opposition. Rooney was a one-man disruption machine yesterday, and the rest weren't far behind.
These tactics are not a surprise, we've seen them many times before. Assuming you don't have the players to go face-to-face in a slugging match and hope for a points victory, then the only way to win in such circumstances is to make the ball do the work.
You need to be like Jackie Chan in a fight scene, moving so fast every punch misses its target. If you pass the ball before every onrushing United player gets to you then you suck the wind out of their sails. Even Rooney would give up after an hour of chasing shadows.
Arsenal have the players to do that, but Wenger bottled it, and in the first half there was only one team at the races and it wasn't Arsenal. They were better in the second, playing at a higher tempo, and the introduction of Wilshire added energy, though he seems to be a bit of a loose canon these days.