Arsenal stunned Manchester United with three goals in the opening 20 minutes as the Gunners won 3-0 and moved up to second in the Premier League table on Sunday.
United had no answer to Arsenal's opening burst as Alexis Sanchez and Mesut Ozil scored twice in 74 seconds.
Sanchez added a third from 18 yards as the Gunners beat United for just the second time in 14 league meetings.
United improved after the break but Arsenal substitute Alex Oxlade-Chamberlain hit the crossbar late on.
Defeat leaves Louis van Gaal's side in third place, behind Arsenal on goal difference.
Arsenal's Champions League defeat at home to Olympiakos in midweek left them in danger of failing to reach the last 16 and manager Arsene Wenger in the face of heavy criticism.
Wenger was forced to defend his selection of goalkeeper David Ospina, describing recent criticism as "not analysis of football". He also called the press "boring".
But his side were anything but in an opening half which saw them tear Manchester United apart, almost at will. With their first real attack Sanchez back-heeled in at the near post after good work from Aaron Ramsey and Ozil, and the Germany playmaker then added the second a minute later after Walcott ran at a struggling defence. Sanchez shrugged Matteo Darmian aside to add his second with a brilliant 18-yard strike which brought up an unwanted record for the visitors, as they conceded three goals in the opening 20 minutes of a Premier League game for the first time. And it should have been four just after the half-hour mark as Ramsey volleyed wide from close range when completely unmarked. With the game won, Arsenal failed to repeat their attacking display in the second half until substitute Oxlade-Chamberlain chipped David de Gea and hit the crossbar in injury time. The Gunners are now just two points behind leaders Manchester City going into a two-week international break which had the potential to be uncomfortable for their fans and for Wenger.