Overloading The Midfield Left Space Out Wide
As mentioned, the inclusion of Oxlade-Chamberlain was surprising but didn't hinder Arsenal enjoying domination of the ball but it wasn't the midfield they dominated - it was the more dangerous area just in front of United's back four.
With Marouane Fellaini further up the pitch supporting Wayne Rooney and being occupied by Francis Coquelin and Ander Herrera tasked with watching the roaming Santi Cazorla, Daley Blind was often tasked with watching not only Mesut Ozil but Oxlande-Chamberlain, Alexis Sanchez and occasionally Danny Welbeck as well. This was due to Welbeck's tendency to drop off and link the play and Sanchez and Oxlade-Chamberlain moving infield into the number ten zone. This meant Valencia and Shaw had no choice but to be dragged out of position to aid Blind and prevent the runs of Sanchez and Oxlade-Chamberlain. This is all well and good as long as the wingers, who are typically very high up the pitch in a 4-2-3-1, track their opposing full backs with discipline. Ashley Young has always been superb in this area of the game, which is why Van Gaal has often trusted him playing as a left wing-back, so Luke Shaw had license to track the runs of Oxlade-Chamberlain because he was well protected from Bellerin. The same can't be said for Antonio Valencia, who was susceptible all night to the runs of Nacho Monreal and this is what brought about the opening goal. With Sanchez and Oxlade-Chamberlain making darting inside runs, Luke Shaw and Antonio Valencia were drawn inside leaving Monreal free to exploit the vacated space and place a shot to put the Gunners 1-0 up.