The 22-year-old goalkeeper has featured in United's opening two pre-season matches in the USA and is eager to be given a chance in van Gaal's first-team setup next term.
However, Johnstone is aware the Red Devils may look to bring in a back-up for David De Gea if Victor Valdes happens to depart Old Trafford this summer and is weighing up whether or not he returns to Preston North End for another loan spell following last season's successful four-month stint with the Lilywhites.
"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."