Harrop enjoyed a dream United debut last month when he scored at Old Trafford in a 2-0 Premier League victory over Crystal Palace.
But the attacking midfielder has now opted to drop to the Championship with Preston, signing a four-year-contract on Friday after the clubs agreed "a significant undisclosed compensation agreement".
Harrop confirmed United did offer him a new contract, but the desire for regular game-time prompted him to move on from Jose Mourinho's side.
"I'm 21 now and I'm confident in myself, I need to play first team football to get myself out there," he told Preston's website.
"That wasn't guaranteed at United, even though they offered me a new contract it was very limited in game time and so I couldn't take that risk at my age.
"I need to be out there, proving myself and doing what I know I'm capable of.
"It's a big thing for me to leave United after the number of years I have been there, but I am confident in my ability and know it’s the right decision."
Therefore Harrop leaves United with a record of one appearance and one goal to his name, having progressed through the club's youth system.
"I left on a high, a dream I had since I was young came true and now I am moving forward," he said.
"I am here now and I am delighted to be here and I will take every step as it comes and do the best that I can."