The Northern-Irishman made his senior debut at Old Trafford back in September 2014 and went on to make 27 appearances under Louis van Gaal.
McNair then joined Sunderland on a permanent deal over the summer after failing to establish himself in Mourinho’s first team squad during pre-season.
And the 21-year-old, speaking to the BBC, shed light on the disagreement with Mourinho that led to his departure from Old Trafford.
“He wanted me to go on loan and I went in to see Jose and just said ‘look, if you don’t let me go on a permanent, I’m just going to stay here,” he said.
“That’s how strongly I thought about it. At the time, it’s what I wanted so I’ve got no regrets. I think he quite liked how I was straight up. I said I wanted to leave, that I didn’t want to go on loan.
“I think he respected that. I told him that I’d been here since I was 12-years-old and club could at least let me do what I want to do.
“I spoke to Ed Woodward and said the same to him. I was refusing to go on loan basically. I just wanted a new challenge and off I went.”
McNair decision is not dissimilar to that of Michael Keane, now of Burnley and recently a subject of interest following his emergence under England manager Gareth Southgate, who boldly decided to abandon his dream of featuring as a regular at Old Trafford in favour of securing regular football elsewhere.
Such a decision has started to bear fruit for Keane, but McNair has struggled to impose himself in similar fashion at Sunderland, managing just nine appearances all season under David Moyes. That said, the youngster has demonstrated on several occasions – with his display at Chelsea back in April 2015 springing to mind – that he can establish himself at this level.