The highly-rated keeper will replace Aaron McCarey, who has been recalled by Wolves, and is in line to make his debut for his new club against Hartlepool on Saturday.
Johnstone has impressed for the Under-21s again this term but Ben Amos' return from Hull City has increased the goalkeeping options open to Warren Joyce.
The England Under-19 international has already had temporary spells with Oldham and Scunthorpe.
"I want the next step of competitive football,” Johnstone told ManUtd.com recently. "I think you want to be playing in front of crowds.
“When you get in the first team, you can say: 'Well, I’ve done this already on loan for half a season or a whole season so it’s not so new to me'. It’s just the way it goes and it helps you improve.”