Manchester City's veteran defender Martin Demichelis was made to feel his age by Marcus Rashford in Manchester United's 1-0 win on Sunday, according to victorious manager Louis van Gaal.

Rashford, 18, left 35-year-old Demichelis on the seat of his pants en route to scoring the game's only goal in the 16th minute as United re-energised their top-four challenge and shredded City's Premier League title bid.

Demichelis was eventually replaced early in the second half, with Wilfried Bony coming on, and Van Gaal, who coached the Argentina international at Bayern Munich, said that the City centre-back had been given a torrid time.

"Rashford is very quick and Demichelis looked like the years are catching up with him," said the Dutchman after his side moved to within one point of City, who occupy the fourth and final Champions League spot.

"He was a very good defender. He was my centre-back in Bayern, but that is the life of football."

Rashford's clinically taken strike, his fifth in eight games, made him the Manchester derby's youngest goalscorer at 18 years and 141 days and earned him fulsome praise from his manager.

"He's a real striker, that's why I let him stay in that position," Van Gaal said. "He can make goals. I like him very much.

"He is also an attacking point and he runs the channels. But still, he is 18. We have to wait and see how consistent he is."

City manager Manuel Pellegrini said that Demichelis had been taken off because he was "nervous" and "not in a good day", but dismissed suggestions that Rashford had been too much for him to handle.

"I don't think so," said the Chilean, whose side now trail leaders Leicester City by 15 points, albeit with a game in hand.

"It was a mistake in the first goal. I don't remember really a lot of options (chances) for Rashford or Manchester United.

"I think they had just one after the goal and nothing more during the whole game."