Manchester United's Phil Jones, left, and Sergio Aguero of Manchester City during their FA Premier League match at Old Trafford in Manchester, April 12, 2015. [Photo/SIPA]

City's heads dropped after going behind and its defense was a mess at times, particularly after captain Vincent Kompany was substituted at halftime with a groin injury.

After a bright start, capped by Aguero's tap-in from David Silva's cross to end the striker's six-game run without a goal, City struggled to match United's energy and drive.

United equalized on its first attack and never looked back, with the tall Fellaini a constant menace alongside Wayne Rooney in attack and the rejuvenated Young providing a regular supply of crosses from the left.

Young equalized from close range in the 14th after Gael Clichy's sliding tackle deflected off winger and landed in his path. He then turned provider in the 27th, sending a curling cross over the head of Clichy for Fellaini to plant a header beyond the outstretched left hand of goalkeeper Joe Hart.

"He was fantastic," Van Gaal said of Young. "He was our best player,"

The second half was even but United had a cutting edge and seized the initiative with two quick goals.

Rooney held the ball up well and fed Mata, who ran through unchallenged to place a finish between Hart's legs in the 67th. City's defending was poor for the fourth goal, too, as Young whipped in a free kick that an unmarked Smalling headed in from eight meters.

Aguero's shot from Pablo Zabaleta's cross was fumbled into the net off the post by United goalkeeper David de Gea, but there was to be no record fifth straight derby win for City.