1.United adopted the wrong tactics
It was plain to see from the very first whistle United were going “gung ho” and decided to adopt a high pressing game to get at Arsenal and hope for an early goal. This wasn’t the case and heavily backfired for United, with the blistering pace of Sanchez and Walcott on the counter proving lethal.
They played right into Arsenal’s hands and perhaps would have been better adopting a more cautious counter attacking game, particularly considering they were away from home. Allowing Arsenal the possession of the ball and picking them off on the counter was a tried and trusted tactic utilised by United under Ferguson and more recently demonstrated by Mourinho at Chelsea.
But It was almost a complete role reversal by United; dominating the ball and being picked off themselves a very strange decision by a tactically capable manager. Van Gaal has a very strict philosophy when it comes to ball retention and it will often work, but his lack of flexibility is a concern.