He was out of contract at Old Trafford but had been offered two extensions by the club.
However, Harrop quit the club for Championship side Preston on a four-year deal.
“Before my debut the plan was for me to go and get games," Harrop said. “Then when I scored it made things a lot harder.
“United had already offered me a deal and they offered me another on top of it which made the decision tougher.
“It would have been easy to take it. There was no guarantee I was going to play though.
“I'm too old to be playing reserve team football and need to playing first-team football but there was no promise of that at United."