De Gea had to be alert to make a double save from Andriy Arshavin and van Persie but the champions carried the greater threat with Welbeck diving to head Rooney's cross off target. A hamstring injury cruelly ruined the young striker's afternoon, and Arshavin, who was very fortunate to still be on the pitch after a foul on Young when already booked, flashed a warning shot over the bar at the other end.
Jenkinson was also lucky to avoid a red card when hauling Young down as he sprinted through on goal but Rooney ensured his own justice was meted out by rolling the free-kick for Young to trap and then sweeping past Szczesny with the power on the shot too much for the Polish keeper.
Rooney showed the confidence flowing through the team when testing Szczesny from his own half but there was real complacency seconds before the interval to allow Tomas Rosicky to feed Walcott, whose drive from an angle whistled through De Gea's legs for 3-1.
It was a message that the job was still far from done, and one that was taken on board as Sir Alex's side turned up the heat again in the second half. Szczesny saved well from Young and Nani chipped wastefully off target as the hosts pressed but the Gunners responded with De Gea superbly stopping a van Persie volley and Arshavin firing wide when Smalling was caught up the pitch.
Tom Cleverley shot weakly at Szczesny after a brilliant one-two with Young and Nani failed to convert a cross from the former Aston Villa winger but the dam broke again when Rooney repeated his free-kick feat in the first half by whipping home for 4-1 and becoming United's all-time record scorer in the Premier League.