As a youth teamer, falling behind in his physical development at United was tough for McTominay. But he had support from his room-mate, Riley, who is now at Bradford City in England's fourth tier after leaving United in May 2018.
"I was in digs with Joe Riley and he was someone I looked at as a man at that age," he told Sky Sports. "I always wanted that, I always wanted to be able to run for days and compete with some big boys.
"Joe was good, he always used to keep me calm and say how well I was doing in training - even if I wasn't doing that well. He would always check on me and see if I was alright and I will always remember that. We lived together for five years and it really set me in good stead."