United made the breakthrough in the 16th minute following a corner from Nani.
Three minutes later United made it 2-0 after good work by Nani on the left.
He chipped the ball into the area and Fletcher marked his first start in two months by heading home, although his effort seemed to take a deflection off William Gallas.
United made it 3-0 in the 38th minute following a pinpoint pass from Michael Carrick from deep.
Nani was alert and skipped goal-side of Hoyte to score with a low drive into the corner.
After halftime, Arsenal's problems increased in the 49th minute when they were reduced to 10 men.
Emmanuel Eboue was dismissed by referee Alan Wiley after a high diving challenge left Patrice Evra on the floor.
Eboue refused to accept the decision and questioned the decision before being led away down the touchline.
Fletcher rounded off the scoring 15 minutes from time after superb wing play from Nani saw the Portuguese sit the ball up at the back post for the Scot to power home past a hapless Lehmann.