The 19-year-old was given a first-team opportunity during United's pre-season tour.
Having experienced what it is like to play senior football, Johnstone has admitted that he is ready to step up again whenever he is needed.
"You have to have a lot of hunger anyway just to be able to want to go up and play, get your chance and do well," he told MUTV. "Having a taste of it makes you hungrier to go for it, give it a good go and do your best. As a goalkeeper, you've got to be patient but, also, your chance can come very quickly so you have to be ready as well.
"Obviously, David and Anders are top keepers. David is young and Anders has got an older head on him but both are playing and doing well, which shows the manager has got faith in them and doesn't have a problem playing whoever.
"But I want to work hard to push those two and show I can step up if I'm needed."
Johnstone has had loan spells with Oldham Athletic and Scunthorpe United.