The Gunners, whose run of 12 successive wins ended with Sunday's 1-1 draw at Anfield, fielded their second team but the difference in class was embarrassing as they strolled to a comfortable win.
United, who pulled off a surprise 1-0 over the London side in the Premiership just ten months ago, have had some close cup tussles with Arsene Wenger's side in recent years but this time it was a one-sided affair.
They fell behind in the eighth minute to a wonder strike from the Brazilian-born striker Eduardo.
Arsenal's precise and clinical passing found a gap in the United defence as Nicklas Bendtner threaded the ball to Eduardo who drilled a spectacular left-foot shot home.
The unselfish Bendtner picked out midfielder Denilson minutes later but he squandered the chance scooping the ball high over the bar from 14 yards.
Australia international David Carney saw his long-range strike flash wide of the far post but United struggled to get out of their own half for long periods as Arsenal's passing game almost turned it into an exhibition match.
The impressive Theo Walcott should have added a second in the 38th minute when he ran onto Abou Diaby's pass, but the youngster pulled his ten-yard shot wide of the target, and seconds before the break Ian Bennett charged off his line to save bravely at Walcott's feet.
Eduardo grabbed his second in the 50th minute, the goal carved out by full-back Kieran Gibbs' long pass downfield.
The ball evaded the slow-moving United defence and the Arsenal striker coolly prodded the ball past the advancing Bennett from 14 yards.
Jon Stead fired over the top before Chris Armstrong blundered in the 65th minute to let in Walcott, the little midfielder outpacing the left-back to drill in a low shot past the advancing Bennett only for the ball to crash off the foot of the post.
Denilson wrapped up the game in the 69th minute, the United defence standing off the Brazilian as he finished with a looping 22-yard shot, the ball glancing off Chris Morgan as it sailed over Bennett's head.
Minutes later a disgruntled fan charged towards United boss Robson, and flung a red and white scarf towards him before being pulled away by stewards.