Midfielder McTominay, who qualifies for Scotland and England, has been called up by Alex McLeish for the former's friendlies against Costa Rica and Hungary.
The 21-year-old has made 18 appearances at Man United so far this season and is rated very highly by manager Jose Mourinho.
"I'm looking forward to seeing how he deals with the national game and hopefully he'll play in the two games that are coming up," McClair told BBC Sport.
"If he continues to do what he's doing for Manchester United, if he keeps playing well, pleasing the manager that he's got, his coach or manager at Manchester United, he'll do well.
"Alex McLeish made a huge effort getting to Carrington to meet up with him, because it was in the middle of the bad weather that we had. He made it, put a case, [England manager] Gareth Southgate sent him a text."
McClair previously worked with McTominay at United's academy.