19-year-old Williams was excellent in the goalless draw with AZ Alkmaar last week.
"I thought he did brilliantly," McTominay told MUTV.
"I said to him before the game to just be himself and told him he had nothing to worry about.
"I told him to go and play the game like it was a Reserves match or he was playing in the park with his mates… you don't want to think too much about it, you just need to go out, express yourself, show what you can do and that's what he did."
A fellow United academy graduate, McTominay exactly what Williams must do from here.
He added: "He just needs to concentrate on how he's performing on the pitch and listen to Ole and the coaches and I'm sure they'll be pleased with him.
"Obviously, we didn't create as many chances as we hoped [against Alkmaar] but we still got a point away from home and didn't concede again so they're positives we can take.
"And, of course, Brandon was a big, big plus and I'm very pleased for him. To have a game like that this early in his career is great."