Mourinho also selected Axel Tuanzebe and Timothy Fosu-Mensah, who both have slightly more first-team experience than their fellow youngsters. The midfielder's body language suggested it was the former and he appeared on the pitch after the final whistle.
"When I just started with the first team everything went well, then I had a period where it was a bit more hard for me", he recalls.
"Everybody knows I support Ajax, that I am from Amsterdam, and that I played there from the age of seven. I am in a final now".
Mourinho prioritised winning the Europa League as United's most likely route to Champions League qualification weeks ago, and the Red Devils coach will simply be hoping no key players are injured before next Wednesday's final against Ajax. "He played 90 minutes in the last match, so probably not".
And more club history was made when Angel Gomes came on as a substitute for Wayne Rooney to become United's youngest debutant since David Gaskell in 1956 and the club's youngest player in the Premier League era.
He beat Federico Macheda's record and is the youngest player since Duncan Edwards. "A pundit is not honest if they can not forget their colours". It's not my fault if their managerial career was very poor.
While Sam Allardyce changed James McArthur for Yohan Cabaye and Patrick van Aanholt for Andros Townsend, Mourinho gave four full debuts.
It was a game that Harrop, in particular, will never forget thanks to his debut goal which followed a hat-trick he scored for United's under-23s in a victory against Tottenham at Old Trafford on Monday.
The manager said he did not know whether England striker Wayne Rooney would feature.
"They're nice words and now I need to prove that he's right", he said.
Jamie Redknapp says there is "no joy" in the Manchester United team under Jose Mourinho.
At the end of the first half, Michael Carrick came on for Pogba and moments later Anthony Martial entered for Jesse Lingard. Sometimes that means they don't have too much pressure. He started playing when he was young and it is an honour to be the youngest since him. He showed what he can do with an acrobatic save from Luka Milivojevic's effort. It was that type of day.
Zlatan Ibrahimovic is out for an estimated nine months after damaging knee ligaments at the team's Europa League quarter-final match against Anderlecht but Mourinho said the Swede would be in Stockholm for the final with the rest of the squad, on crutches or not.