The England star didn't make his debut until 2016 but has enjoyed a meteoric rise in the years that have followed.
He was given his bow by Louis van Gaal in a Europa League clash but had been drafted into first-team training by former boss David Moyes.
"It was a group of us that went over. It was an amazing feeling," he told ESPN FC.
"I remember the training session: We didn't actually touch the ball. We were just doing shape and stuff like that. But then we go back to our own age group, and what you learned from that little 15 or 20 minutes is priceless as a young academy player.
"Those moments really do start to add up, and when you start training with them more regularly, you pick up more things, more things to learn from. That's how you become a first-team player."