Arsenal 1-2 Manchester United
Danny Welbeck's late strike gave Manchester United victory at Arsenal and kept them three points behind Premier League leaders Manchester City.
After a quiet opening Ryan Giggs picked out Antonio Valencia to head United in front at the far post.
Arsenal improved after the break and although Robin van Persie missed a clear chance, he made amends by finding the bottom corner from 15 yards out.
But United won it as Valencia ghosted through and Welbeck lashed home.
A third successive defeat was harsh on the Gunners, who were in the ascendancy for much of the second period.
Whilst there was never any danger of a repeat of their , defensive concerns were evident again and they remain in danger of not qualifying for the Champions League, in which they have competed for the last 14 seasons.
Two of the great returning characters in the serial drama between two sides with 15 Premier League titles between them did not have lead roles, Thierry Henry failing a fitness test and Paul Scholes on the bench.
Arsene Wenger focused on his younger generation, former Southampton widemen Theo Walcott and Alex Oxlade-Chamberlain supporting Van Persie in attack against a United defence missing Rio Ferdinand because of back trouble.
The visitors were given added inconvenience defensively in the early stages by an injury to Phil Jones, who was stretchered off after twisting his ankle.
Despite the occasional burst from the lively Oxlade-Chamberlain the opening half an hour was played at a surprisingly low tempo.
With Arsenal unable to find their fluency it was United who began to pose more threat with their patient build-up play.
Patrice Evra eluded Alex Song and cut back from the byeline to Nani whose first-time effort from 10 yards was well kept out low to his right by Wojciech Szczesny.
Nani then exposed Johan Djourou's positional weakness down the United left, but produced neither a telling pass nor shot as the ball drifted harmlessly across goal.
However, United were ahead when Giggs was given time to curl in a delicious centre that Valencia attacked with more vigour than Thomas Vermaelen, again employed at full-back.
Such was Wenger's concern with Djourou on the opposite side that he brought on 18-year-old Nicholas Yennaris to replace him at half-time.
A slip by Chris Smalling should have given Arsenal an equaliser, Tomas Rosicky unselfishly laying the ball back to Van Persie, who weaved round two defenders but blazed his shot wide.
Arsenal finally began to construct some useful passing patterns and after Oxlade-Chamberlain squared to Aaron Ramsey the Welshman jinked into a shooting position, only to fire over.
United still presented concerns to Arsenal's fragile backline and Welbeck outpaced Per Mertesacker.
The Arsenal defender appeared to clip Welbeck outside the area but the England striker stayed on his feet and touched past Szczesny, though Mertesacker was able to hook off the line.
A classically constructed move then brought Arsenal level.
Laurent Koscielny calmly played his way out of defence and found Rosicky, who picked out Oxlade-Chamberlain with a crossfield pass. The winger jinked inside and delivered a perfectly-timed pass for Van Persie to dispatch.
To the dismay of the Arsenal faithful, who could sense victory, Oxlade-Chamberlain was withdrawn in favour of the rather less popular Andrey Arshavin, and the Russian did nothing to endear himself by failing to halt Valencia's marauding run into the box, with Welbeck slamming home the loose ball nine minutes from time.