McTominay has emerged as a a surprise key player for United in recent months, earning the trust of Jose Mourinho after breaking into the first team.
"What's he playing at?" the former Newcastle star said.
"He obviously wants his summers off", Barton said, per the Daily Record.
"He has shown his ambition hasn't he?"
And, according to The Times, the Man United youngster has come to a decision.More news: World pushes back on Trump's trade tariffs
"It would have been easy to go financially to the England side and, if you are in the Manchester United team, then there's a good chance you get called up by England".
"That's been a route map for a long period, as you see with players like Marcus Rashford and Jesse Lingard".
"So fair play to the boy if he feels like a passionate Scotsman, it can only be a good thing for Scotland because he's a seriously talented player".
"Who will I support in the World Cup qualifiers?"
McLeish is expected to now include the 21-year-old in his squad for the friendly double-header against Costa Rica and Hungary at the end of March.
McTominay qualifies for Scotland through his grandparents, but had resisted attempts to persuade him to pledge his allegiance to the Scots, preferring instead to focus on establishing himself in the senior squad at Old Trafford.
Scotland have not qualified for a major global tournament since the 1998 World Cup, while England have reached every event since, except Euro 2008.