McTominay has made 15 appearances for the Red Devils this season, establishing himself in Jose Mourinho's team, at times at the expense of Paul Pogba.
England had expressed interest in his services, but the 21-year-old has committed his future to his ancestral home, even though he is yet to represent Scotland at any level but has attended training camps with youth sides in the past.
The player's grandfather, Frank, revealed Ferguson was a key figure in McTominay's development at Manchester United and, alongside current manager Mourinho, pushed him towards rejecting the country of his birth for Scotland.
"Alex McLeish handled the situation quite well," he told the 'National'. "It's been rumbling on now for two or three months now in the papers. But he didn't go shouting and bawling. He just said that he would speak to Scott and he did. He conducted himself quite well."
"But Sir Alex Ferguson was keen for Scott to play for Scotland as well. He was up front about it. I know that for a fact. I can say that without fear of getting contradicted. That has been kept under wraps. He and Jose Mourinho were both quite keen on it over the last month or two."
"Sir Alex Ferguson has been very good with Scott over the years. He gave him his first contract with Manchester United when he was 16 before he retired and then made sure it was extended when he was 18. He seems to get on well with him. I have seen them talking."