New signings Sofyan Amrabat, Mason Mount and Rasmus Hojlund have been named in the group as expected, along with Jadon Sancho even after his very public disagreement with Erik ten Hag over his performance in training.
There are places too for Jonny Evans and Sergio Reguilon, as well as Harry Maguire and Scott McTominay despite both being strongly linked with moves away from Old Trafford over the summer.
That is because Under-21 players can be registered on the B List, which will be submitted ahead of every Champions League match and includes homegrown talent.
Garnacho and Mainoo are among those who will be on that list when United confirm the squad in advance of their opening Group A clash, a daunting trip to face Bayern Munich.
Erik ten Hag’s side then take on Copenhagen and Galatasaray, and will hope to have their place in the knockout stages sealed by the time Bayern travel to Old Trafford on December 12 for the final group-stage encounter.
Garnacho can be expected to play an important role in United’s Champions League campaign, having started two Premier League games this season before impressing off the bench against Arsenal.
The 19-year-old appeared to have won the match for United in north London, holding his nerve to finish past Aaron Ramsdale in the closing stages, but the goal was ruled out by VAR and the Gunners went on to score twice in added-time.