Pierre-Emerick Aubameyang's second-half penalty, conceded by Paul Pogba, gave Arsenal a 1-0 win in a drab game with Manchester United at Old Trafford.
The Gunners skipper sent David de Gea the wrong way after Pogba tripped Hector Bellerin in the 69th minute to settle a game that rarely looked likely to provide a goal from open play, rewarding Sporting Life followers who took Mark O'Haire's advice and backed under 2.5 goals.
United substitute Donny van de Beek did hit the woodwork late on, but an equaliser would have been more than Ole Gunnar Solskjaer's team deserved for a lifeless display in what was his 100th match as Red Devils manager.
Victory for Arsenal was their first at Old Trafford since 2006, and ended a 29-game run without an away league win at a fellow Big Six club.
For United, October started with a home humbling at the hands of Tottenham and now November has kicked off with another loss to north London visitors; Solskjaer's men fell well short of the standards reached in midweek against RB Leipzig.
It was quite the comedown from that five-star Champions League demolition, with defeat meaning they have failed to win in their opening four home league games for the first time since 1972-73.
Few would argue Arsenal did not deserve to win after edging a dull first half in which Willian clipped the crossbar.
United’s sloppiness was encapsulated by the poor Pogba challenge on Bellerin that allowed Aubameyang to score from the spot, with Arsenal battening down the hatches – and riding their luck at times – to win.
The Gunners had the best chances of a quiet first half as Willian skimmed the top of the crossbar after playing a smart one-two with Aubameyang and Bukayo Saka missed the target with a close-ranger header that Aaron Wan-Bissaka gifted him.
United returned from the break with renewed vigour but a familiar carelessness, and it was Arsenal looking the most likely to score as the impressive Thomas Partey and Mohamed Elneny tried their luck.
The visitors would eventually open the scoring in the 69th minute after Pogba made a penalty-box challenge on Bellerin that was as clumsy as it was needless.
Referee Mike Dean had no hesitation pointing to the spot and Aubameyang coolly sent De Gea the wrong way.
Solskjaer turned to his bench in a bid to eke out a leveller and Gabriel looked fortunate to avoid picking up a second yellow card for a challenge on Greenwood.
Nemanja Matic then claimed the Brazilian caught him in the box and fellow substitute Van de Beek saw a shot deflect off Elneny on to Leno’s face and back off a post.
The hosts continued to push but could not find a way through.