The summer signing from Swansea got his United career off to the best possible start with a goal on debut against Chelsea last weekend.
And McTominay was delighted to see his new friend perform on the biggest stage.
The Scot said: "I've had a really really close bond with him since he came to club.
"I knew a bit about him before he came and I am just so, so pleased.
"It's probably one of the happiest I've been on a football pitch - seeing one of my pals score his first goal for the club.
"I was made up for him so when I got home I sent him a text and said well done.
"He deserves it. He's such a hard working lad and he knows it's only the start.
"It's not just one goal, it's about getting 50 or 100.
"He's very capable of that and as you've seen he's done really well at the start of his Man Utd career."