The 16-year-old was named in Jose Mourinho's squad for the pre-season tour after a remarkable run of form.
And now he has made it onto the bench for the huge Champions League game at PSG.
Who is Mason Greenwood?
The striker could be Manchester United's best-kept secret.
Greenwood was born in Wibsey on October 1, 2001 and he joined United in 2018.
He began his career as a midfielder, but has since evolved to become a striker.
In 17 matches for United's youth team last season, Greenwood scored 17 goals.
Has Mason Greenwood played for England's youth teams?
He made six appearances for England's Under-17 team last season.
And he was part of the squad in the Algarve Tournament in Portugal earlier this year.
What has been said about Mason Greenwood?
Manchester United legend Clayton Blackmore said: "We don't know how good he is going to get. He's still growing.
"He's great on the ball and very good with both feet.
"He's the first person I've seen that takes penalties and free-kick with his wrong foot.
"I've never seen anything like that! He's got a great chance."
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