The highly-rated keeper had a taste of life in Sir Alex's side during the DHL pre-season tour and has started the campaign in impressive form at Under-21 and Reserves level. Although still a teenager, the Preston-born shot-stopper appreciates he needs to be ready for when his opportunity comes.
"You've got to up your performance with the first-team so it helps you step up," explained Johnstone to MUTV. "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.
"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."
The 19-year-old is also determined not to rest on his laurels after helping the Reserves sweep the board in terms of trophies in 2011/2012.
Warren Joyce's side have continued that form into this campaign by topping the Barclays Under-21 Premier League table and winning both Manchester Senior Cup ties.
"The lads have to realise we've done that but that's gone," he insisted, when asked about the