The Argentina-Spain winger came off the bench in last Thursday's 1-1 draw with Chelsea at Old Trafford, having been an unused substitute in several previous games.
"Incredible moment making my debut at 17 years old for Manchester United in the Premier League at the Theatre of Dreams," Garnacho tweeted.
"I can’t put into words what I’m feeling right now, I’ve been dreaming of this moment since I was four years old. Remember guys, Dreams really do come trueu2764ufe0f"
Here's everything else you need to know from the last seven days...
IN CASE YOU MISSED IT
Defeat for U23s
Our Premier League 2 trip to Merseyside ended in defeat.