United came from a goal down to cruise to the win, with Greenwood hitting a superb second-half strike.
For the tenth time this season, United had come from a goal down to win and Greenwood said: "It is some achievement. We don't really want to go behind but it gives us a little boost to come back and get the win.
"It was well improved [in the second half]. Ole [Gunnar Solskjaer] said his bits in the changing room. It seemed to pay off.
"To be on the main stage and scoring important goals like today, it means a lot.
"We just concentrate on ourselves, we take it game by game. We want to get as many points as we can and keep the pressure on."