The 18-year-old used his lightning pace to run behind the defense before rifling an unerring finish into the roof of the net after 17 minutes.
The teenager wheeled away in celebration before being mobbed by his teammates, including Cristiano Ronaldo who had provided the assist.
Then came a moment that particularly pleased Manchester United fans.
After appearing to ask Ronaldo, the youngster turned to the supporters and adopted a pose which the 37-year-old has recently used as his own celebration – clasping his hands over his chest and looking skyward. Cue instant comparisons.
“18 years and 125 days dreaming of this moment. Thanks Idol, @Cristiano,” Garnacho tweeted after the game.
The youngster, who joined United’s youth academy in 2020, received a standing ovation from United fans as he was substituted in the second half.
Many had been calling for Garnacho to play more this year after impressing in preseason but United’s star midfielder Bruno Fernandes said the youngster still had to improve on certain aspects.
“He was good, but he knows we expect a lot from him,” Fernandes told BT Sport after the win.
“In the [pre-season] tour, he didn’t have the best attitude. That’s why he didn’t get chances. Now he’s getting his chances because he’s training better, has a different attitude, and he deserves it.”
Despite Garnacho’s goal handing his team the win, it was ultimately a disappointing night for the Red Devils who needed to win by two goals to overtake Real Sociedad at the top of the group.
It means United, which finished second, will are likely to face a tougher opponent in a Europa League play-off match for a place in the round of 16. The draw will take place in Nyon on Friday November 24.
Garnacho, though, was a bright spark in a decent performance from Erik ten Hag’s men who will look to take the winning form into the English Premier League.
“I am really happy and I hope he can keep it going,” Ten Hag said of Garnacho.
United play Aston Villa in its next league match before facing the same side again during a midweek League Cup fixture.