Mikel Arteta’s men were beaten by Chelsea last time out, but they showed no signs of a hangover as they romped to victory at the Emirates Stadium.
Despite not winning any of their last five matches, the Gunners made a blistering start and opened the scoring inside the first eight minutes.
Sead Kolasinac’s run down the left was not tracked, and his cutback found Pepe, via a deflection off Victor Lindelof, and the winger smashed the ball into the corner.
United failed to respond and could have fallen two-nil behind when Lucas Torreira tried his luck in the 32nd minute. The midfielder turned in the box and aimed his shot towards goal, but he could only fire past the near post.
Six minutes later Pepe rattled the upright from the edge of the area after goalkeeper David de Gea’s poor clearance, but the Red Devils got away with it.
However, it was soon two-nil to the Gunners after some more poor marking from a corner. Lacazette’s header was saved by De Gea, but his clearance hit Lindelof and Sokratis was on hand to volley home from point-blank range.
In a desperate bid to equalise, United sent Harry Maguire forward late in the half, but the England international could only steer a header into the hands of Bernd Leno as the half-time whistle sounded.
In the second-half, Arsenal failed to test De Gea, but they had done enough as they tried to see the game out. Dan James tried his luck at the near post but was denied by Leno in the 56th minute before Andreas Pereira stepped off the bench and found the side netting five minutes later.
Ole Gunnar Solskjaer’s men looked out of sorts as they failed to find a way back into the game, with their final chance falling to Marcus Rashford but Leno denied him after rushing out of his box.
Arsenal’s win gives them the confidence they can still qualify for Europe, while United will need to go back to the drawing board after another flat display.