Mason Greenwood, 17, opened the scoring from close range after two minutes.
Marcus Rashford added another before half-time, when manager Ole Gunnar Solskjaer made 11 changes.
Phil Jones' powerful header made it three, and Anthony Martial's penalty rounded off the scoring in front of a crowd of 55,274.
Solskjaer's side were without striker Romelu Lukaku, who is still to make an appearance on this trip after picking up 'a knock' in training on Tuesday.
Mason's the man
The Belgian's absence will only intensify discussion about his future.