The attacking midfielder scored in his one appearance for the first team at United last season when Mourinho rested his min squad ahead of the Europa League final.
Preston have signed two midfielders from United in recent season and it has been hit and miss. Liam Grimshaw has failed to progress after signing when injured. He was played out of position at the start of last season and never really recovered before being sent to Chesterfield on loan.
Ben Pearson has had an opposite impact at the club. Whilst he was not in the team at the start of last season he soon became the first name on the team sheet with thew combatitive midfielder an integral part of the side.
Preston will hope that Harrop can add goals to a side which shares around the goals. If he can impress as much as Pearson then the midfield will be an exciting place next season.