Fosu-Mensah was handed his Man United debut by van Gaal as a substitute in the 3-2 win over Arsenal in February.
The 18-year-old revealed that a number of his teammates helped him to settle his nerves when he came on against Arsene Wenger’s side.
“I was overwhelmed, very overwhelmed when I came on,” he told ManUtd.com.
“When I was sat on the bench, I was more nervous than when I had to play football itself for the first time. Thankfully, the players around me helped me very much. Michael Carrick, Daley Blind and Guillermo Varela really helped me to play well and do what I had to do.
“After the game, Carrick came up to me plus Memphis and Blind. The manager also came to me and congratulated me. Ryan Giggs as well. Everyone told me I’d done well and it was very pleasing for me.”
Fosu-Mensah then completed the full 90 minutes in central defence as United beat Watford 1-0 a few days later.
“It was my first start but I think, as I’ve said, the players around me helped me out,” he added.
“For a young boy of 18 to be put in the team for such a big game shows confidence from the manager in me. So I wanted to pay that back with a good performance.
“Obviously, I will keep working hard. As a young player, you have to do that and keep listening to all the advice from the manager and coaches, especially if you want to play more games. And when the chance comes, you have to play well and make the most of it. I’m happy but, as I’ve said, I think I have to keep my head down and keeping working on my game.”
Fosu-Mensah has enjoyed a superb start to his United career but he still has plenty of work ahead of him.
Louis van Gaal will no doubt keep his feet on the ground as United approach the final part of the campaign but it is hard not to get swept up in the romanticism of a young player flourishing in the first team.