Greenwood also won a penalty which Juan Mata converted to earn United a 4-0 win that meant they qualified as group winners.
The brace took Greenwood's total to six goals in a breakthrough season for the 18-year-old.
Ole Gunnar Solskjaer has been full of praise for the young striker ever since his emergence in the first-team set-up.
And now the United manager has revealed how he first encountered Greenwood when his eldest son Noah was training at United's youth academy.
"I actually do remember when I met him for the first time because my son was training at the Cliff as an eight-year-old and Mason was seven," said Solskjaer.
"The first time I saw him he stood out and I took a picture with him. Now he's here.
"I asked him (for the photo) because I knew he was going to be a player. He has probably got it or his dad has probably got it.
"I do remember the kid because I remember talking to Noah about that. So when he signed his contract we had a little nice memory."
Greenwood's presence in the United team is symbolic of the club's tradition of playing youth.
The game against Everton on Sunday will mark the 4000th consecutive game where a player from United's academy has featured in the first-team.
The record goes all the way back to 1937 and Solskjaer recognises it as something to be proud of.
"I don't have enough players to not play academy players," he said.
"It's something we're proud of, it's in our DNA.
"This club is built on having your own local lads or academy players coming through. We pride ourselves on that. You can see today, the academy players know what it means."
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.