The 20-year-old left United’s youth set-up at the age of 14.
Making the tough decision to quit a potential route to stardom at Old Trafford, he took a scholarship in Massachusetts.
Despite being somewhat of a late developer, the move has been vindicated.
Harrison joined MLS side New York City two years ago and his performances under Patrick Vieira’s guidance earned him an England Under-21s call up this week.
The winger is now reportedly being watched by Manchester City and former club United.
And Harrison has revealed how the Red Devils were gutted to see him go.
“My mum [Debbie] came up with the idea of me going to boarding school in America to improve my education while I played football,” Harrison told The Telegraph.
“I was really apprehensive at first and didn’t want to leave because everyone in that system is just tunnel-visioned and just think about making it to the first time and a lot of players don’t.
“United were disappointed but I don't regret going out to America at all.
“I have not only learnt a lot there about football but I have learnt a lot about life in general. I'm a well-rounded person, being at all these different places and meeting different people.
“It has definitely helped me become who I am today and, luckily for me, it’s all worked out."
On the difference between football in England and soccer in the US, Harrison added: “A lot of people look down on the MLS from here, having the Premier League, which is one of the best leagues in the world, but in reality it’s not that different to a Championship side.
“All the guys out there are really athletic and then on top of that you have players who have played in the Premier League before and had that experience under their belt. The league is developing rapidly.
“Playing with Pirlo, Villa and Lampard, and learning off Vieira, you learn so much about things on and off the field.”
Harrison could feature on Friday when England U21s host Scotland U21s at Middlesbrough's Riverside Stadium, before playing in Andorra on Tuesday.
