Antonio Valencia's opener was cancelled out by Robin Van Persie but the visitors struck late to inflict a third straight Premier League defeat on Arsene Wenger's team
After Robin Van Persie had cancelled out Antonio Valencia’s first half goal, Welbeck struck nine minutes from time to secure the points.
The result pulls the defending champions to within three points of title rivals City, with Arsenal remaining in fifth place following a third straight league defeat.
Arsene Wenger handed Alex Oxlade-Chamberlain a start on the wing, preferring the 18-year-old to Andrey Arshavin, who was forced to settle for a place among the substitutes. Tomas Rosicky replaced the injury Mikel Arteta in midfield, with a calf strain ruling Thierry Henry out of the matchday squad.
Rio Ferdinand, who was suffering from a back injury, and Rafael were withdrawn from the United defence, with Chris Smalling and Phil Jones reinstated alongside Jonny Evans and Patrice Evra. Ryan Giggs replaced fellow veteran Paul Scholes in midfield.
Unfortunately for the away side, Jones did not last long. Facing down Theo Walcott on the Arsenal left, the young defender appeared to injure his ankle by tripping over his own feet and was stretchered off, with Rafael returning to the fray in his place.
Gradually, after a lukewarm first 30 minutes, the visitors began to assert more authority over proceedings. Wojciech Szczesny was forced into a good save when Nani sent a firm shot towards the bottom corner after Evra had shaken off Alex Song in the box.
In first half stoppage time, they made their possession count. Giggs, playing with the energy of a midfielder 15 years younger than his 38, overlapped beyond Nani and whipped an inch-perfect cross to the back post that Valencia rose to head past a stranded Szczesny.
Johan Djourou, playing out of position at right-back, had badly struggled during the first frame, with the majority of United’s best attacking work coming down their left-hand side. Despite the absence of any obvious replacement Wenger did not hesitate in making a change at the break, handing 18-year-old Nicholas Yennaris his Premier League debut.
Arsenal were to spurn a glorious chance to level the scores early in the second half. After Smalling slipped in possession on the halfway line, Rosicky took advantage to steal in and break down the left with the support of Van Persie. Inexplicably, after receiving the Czech midfielder’s pass, the Premier League’s top scorer slammed a shot from no more than ten yards out wide of the post.
Aaron Ramsey then fired over the crossbar from a good position after skilfully working his way into the penalty area as the Gunners continued to fail to test goalkeeper Anders Lindegaard.
The signs were encouraging, though, and the home side were beginning to grow in confidence. Rosicky’s likely goal-bound, stabbed effort was blocked before Oxlade-Chamberlain shot narrowly wide from 20 yards after adventurous play from Laurent Koscielny.
The more men Arsenal committed forward, of course, the more potent Manchester United’s counter-attack was likely to become and Wenger’s team had to survive one such close encounter after 63 minutes. Latching on to a clearance behind Per Mertesacker, Welbeck just did enough to poke the ball past Szczesny but the German defender had enough pace to chase back and clear the trickling shot away.
An excellent challenge from Koscielny spoiled another such United break and launched one of Arsenal’s own. The impressive Oxlade-Chamberlain broke inside into space and slipped a through ball into the path of Van Persie, who made up for his earlier miss with a perfect finish across goal.
Surprisingly, Oxlade-Chamberlain was substituted shortly after the goal, to the displeasure of the Arsenal supporters, who were perhaps further antagonised by the introduction of the enigmatic Arshavin.
The decision was to backfire badly. Arshavin was guilty of some woeful defending as Valencia skipped past the Russian and into the penalty area, exchanging a quick one-two with substitute Park Ji-Sung before teeing up Welbeck, who sealed the points from ten yards.