Manchester United manager Louis van Gaal says he had no option but to start Marcus Rashford against FC Midtjylland in their last-32 Europa League tie.
The 18-year-old forward made his debut after Anthony Martial was injured in the warm-up and scored twice in a 5-1 win as United won 6-3 on aggregate.
Rashford's name was not listed in the United squad in the programme.
"I wasn't worried about him. I'm always confident for that," said Dutchman Van Gaal. "He was the best option."
The hosts, who suffered a shock 2-1 defeat in the first leg in Denmark, fell 3-1 behind with Pione Sisto's effort and, after pulling one back via Nikolay Bodurov's own goal, saw a Juan Mata penalty saved.
However, Rashford scored twice before an Ander Herrera penalty and a Memphis Depay strike moved United into Friday's last-16 draw.
|More on the Europa League|
|Man Utd 5-1 FC Midtjylland - match report|
|Who is Marcus Rashford? Two key people from his development explain|
|Football Daily podcast: 'Rashford like Van Nistelrooy'|
|Liverpool 1-0 Augsburg|
|Tottenham 3-0 Fiorentina|
Manchester-born Rashford used to play for local club Fletcher Moss Rangers as a junior and made his way through the youth system at United.
Former Old Trafford players Wes Brown, Danny Welbeck and Ravel Morrison represented Fletcher Moss Rangers, as well as current United defenders Cameron Borthwick-Jackson and Tyler Blackett.
Rashford had not started a game above under-19 level for United and his most recent goal was for the club's Under-18s in mid-February, which helped lift them off the bottom of their youth league.
|Four more dream United debuts|
|Wayne Rooney - scored a hat-trick against Fenerbahce in the Champions League in September 2004.|
|Federico Macheda- scored a late winner after coming off the bench against Aston Villa in April 2009.|
|James Wilson - scored twice in a 3-1 win at home to Hull in May 2014.|
|Anthony Martial - came off the bench to score in a 3-1 win against Liverpool earlier this season.|
Analysis - Focus on youth
BBC Sport's Simon Stone at Old Trafford
At a time when debate is raging over the likely arrival of Jose Mourinho, who had no track record of picking the kids at Chelsea, a text to my mobile from someone loosely connected with United summed up the irony of what was unfolding: "The importance of giving youth a chance, hey..."
On the pitch at various points this evening were Guillermo Varela (aged 22), Regan Poole (17), Joe Riley (19), Andreas Periera (20) and Jesse Lingard (23). On the bench were Donald Love (21) and Joel Pereira (19). Cameron Borthwick-Jackson (19) was sitting in the stand injured.
That is without mentioning two-goal Marcus Rashford (18).
These are all home-grown players. All, with the exception of Lingard, were given their debuts by Van Gaal.
His choice was not extensive, but when Martial was forced to pull out of the game after tweaking his hamstring in the warm-up, Van Gaal had no hesitation in turning to an 18-year-old.
Trusting youth in such a manner is exactly what the traditions of the club are supposed to be about. Watching from the stands, executive vice-chairman Ed Woodward would have known this.
The walls of the suites within the stadium are littered with faces of the Busby Babes, of George Best, Ryan Giggs, David Beckham and the rest.
If there is a reason why Van Gaal has lasted through such a traumatic campaign, and now seems likely to reach the season's end, this could well be it.
And before Mourinho comes in, it needs to be carefully considered what could be being thrown away.
How bad is Martial's injury?
France international Martial pulled up in the warm-up with a hamstring problem to join an injury list which now includes 13 players.
"Anthony can be tired. He's only 20 and he's played all the matches so it's not so remarkable. He stepped down himself and that I like very much," said 64-year-old Van Gaal.
"I don't think he has a muscle injury. He is very intelligent - he steps down when his hamstring is tight.
"You have to cope with that. It's difficult also for the team because after [first-choice goalkeeper] David de Gea, now Martial.
"Martial and De Gea have played very good seasons and they have to be replaced - but we are replacing them because we are in the next round."
Anthony Martial (hamstring), Will Keane (groin), Cameron Borthwick-Jackson (unspecified), David de Gea (knee), Wayne Rooney (knee), Matteo Darmian (dislocated shoulder), Marouane Fellaini (muscle injury), Adnan Januzaj (hamstring), Ashley Young (groin), Bastian Schweinsteiger (knee), Phil Jones (ankle), Antonio Valencia (foot), Luke Shaw (broken leg).