Arsenal have been spanked yet again by an arch title rival.
A Nani-forced Almunia own goal and Rooney strike put United well ahead at half-time before Park Ji-sung closed the curtains with a goal after the break.
Thomas Vermaelen did pull one back for the Gunners late on, but it was no more than a consolation.
Both sides looked dangerous on the counter early on, but it was United who made the most of their opportunities as they broke the deadlock after 33 minutes.
The impressive Nani did brilliantly to twist his way inside two defenders and get to the dead-ball line.
From there he looked to stand a cross over the head of Almunia, but the Spanish shot-stopper got a hand on the ball and turned it into his own net.
United were now rampant and added a second four minutes later as Rooney and Nani broke at pace and the former rounded off a sweeping move by steering the ball into the bottom corner.
Sir Alex Ferguson’s side were in no mood to take their foot of the pedal after the interval and added a clinical third on 52 minutes.
Arsenal were ripped wide open once again and Park was left with the simplest of tasks as he bore down on goal and slid the ball calmly under the onrushing Almunia.
With 10 minutes remaining the Gunners finally found a way through but an acrobatic volley from Vermaelen, which took a big deflection off Jonny Evans, failed to change the course of the game.