Liverpool came from a goal down to beat Burnley 3-1 and cut the gap on leaders Manchester City to two points, Tottenham Hotspur climbed to third with a 3-1 win over Southampton and Chelsea suffered a shock 2-1 defeat at Wolverhampton Wanderers.
Storm clouds have been gathering over Jose Mourinho's misfiring United side, but goals by Anthony Martial and Jesse Lingard at least ensured they did not suffer a first home league defeat by the Gunners since 2006.
Arsenal, who are now 20 matches unbeaten in all competitions since a shaky start to the season under new manager Unai Emery, led when United keeper David De Gea allowed Shkodran Mustafi's weak header to cross the line.
The visitors went ahead again through Alexandre Lacazette but surrendered the lead less than a minute later to Lingard's strike as they ended the night out of the top four.
Second-placed Liverpool confirmed their best ever start to a top-flight campaign as they hit back from a goal down at struggling Burnley.
James Milner levelled for the Reds after Jack Cork's surprise opener and substitute Roberto Firmino set Liverpool on course for a 12th win in 15 league games with his first touch.
Tottenham Hotspur shook off their north London derby defeat by Arsenal to move into third spot with a 3-1 victory over Southampton at Wembley -- Harry Kane, Lucas Moura and Son Heung-min all on target for Mauricio Pochettino's side.
City, who won at Watford on Tuesday, have 41 points to Liverpool's 39 with Tottenham, who are yet to draw, on 33.
Chelsea are fourth with 31 points, above Arsenal on goal difference, despite losing 2-1 at Wolves who came from behind with quickfire goals by Raul Jimenez and Diogo Jota to claim a first victory in seven league games.
Fulham manager Claudio Ranieri welcomed his old club Leicester City to Craven Cottage and shared the spoils in a 1-1 draw while Everton were held 1-1 at home by Newcastle United.
Southampton, Burnley and Fulham occupy the three relegation spots with nine points each. (Reporting by Martyn Herman, editing by Ed Osmond)
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