The Gunners scored three goals even before the 20-minute mark and held strong from a resurgent Red Devils side in the second half to send them second in the table, two points behind leaders Manchester City.
Chilean Alexis Sanchez opened the scoring with a brilliant back-heel flick in front of David de Gea from Mesut Ozil's cross in the sixth minute, after an incisive build-up and sequence of failed clearances by United's beleaguered defence carved an opening.
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Arsenal hit United again with aplomb some 70 seconds later with an excellent counter-attacking move. Ozil found a surging Theo Walcott, who in turn returned the favour and set up the German to finish neatly past a stranded De Gea.
United’s midfield was very slow to react against Arsenal’s impressive pressing, but it was the performance of summer signing Matteo Darmian which was a real concern.
He struggled to cover the excellent Sanchez throughout the first half, particularly evident when the winger skipped past the non-existant Italian and drove a fierce shot into the top corner, his second of the game.
Darmian was replaced by Antonio Valencia at half-time, the second time this season he’s been replaced by Louis van Gaal at the half.
It doesn’t look good for United or the Italian, who has struggled largely struggled to come to grips with English football in the bigger games, whilst impressing against lesser teams.
Cech the saviour
If it was Arsenal’s forwards that set up the victory, it was goalkeeper Petr Cech that ensured it.
Just before the break, he saved in-form teenager Anthony Martial's strike from close range - had he not, United would have been back in the game.
The Czech Republican also made a brave save from Bastian Schweinsteiger in the second half when he got down quickly to smother the ball at the German's feet.
The veteran keeper made a string of decent saves from Wayne Rooney and Ashley Young to keep a clean sheet, raising further questions as to why he wasn’t played on Tuesday against Olympiacos.
What now for Van Gaal?
The international break will give Van Gaal time to assess why his team started so slowly in a big game and make alterations.
For the Gunners, it’s a great victory to go into the break from and leaves them in a great position, just two points behind City after a seemingly poor start to the season.