Per Mertesacker is indispensable
Succumbing to a virus the morning of the match, Per Mertesacker was forced to sit out the game - only the second Premier League game he's missed this season.
Although Thomas Vermaelen came in to the team and did well, Arsenal clearly missed the domineering presence of the German international at the back.
Unsurprisingly, Van Persie's goal came from the near post - a position usually occupied by Mertesacker. His absence left a gaping hole in the Gunners defence and Manchester United took full advantage.