Johnstone enjoyed a successful loan spell at Preston last year, helping them win promotion to the Sky Bet Championship via the play-offs.
The 22-year-old has played the first two games of United’s pre-season tour and, with uncertainty surrounding the futures of Spanish duo David de Gea and Victor Valdes, Johnstone is hoping Louis van Gaal may give him an opportunity to feature in the first-team squad next season.
“If I go back out I would like it to be a loan,” Johnstone said. “I have to see what happens here first. The manager here gives kids opportunities.
“I have been at this club since I was 10 years old so you have to look at it, speak to the right people and see what the club want from me.
“If they tell me it is time to move on then it is time to move on, but I will keep working hard here and impressing the manager.”
North End manager Simon Grayson recently made it very clear he wants to bring Johnstone back to Deepdale this season.
“We will be patient. I think Sam is certainly worth waiting for,” the Preston boss said recently.
Those comments have gone down well with Johnstone who enjoyed his loan-spell under Grayson.
“I saw what Simon Grayson said. It was good,” Johnstone said. “I enjoyed my time there. The manager, the lads, the fans and the goalkeeping coach were all great.
“I never expected to be going to Preston permanently at the end of the season. There was talk about it. That is between the two clubs to decide.”