The 22-year-old has rejoined his hometown club on a one-month loan deal.
Johnstone helped Preston secure promotion to the Championship last season.
“I’m delighted to be back and hopefully I can start playing some games,” he told Preston’s official website.
“It’s great to be around the team at Manchester United, they are the biggest club in the world but, at the end of the day, I’m 22 years old and I have to think about myself a little bit and I need to be playing first-team football.
“I get to do that now in the Championship, which is a great league to play in.”