Johnstone experienced first team action on the club’s DHL pre season tour, but is yet to make a competitive appearance for United.
Aged just 19 he has time on his side but accepts that he faces a huge task in supplanting David De Gea and Anders Lindegaard between the sticks, thus aims to be ready when ever Sir Alex does call on him, to show just what he can do.
Speaking to MUTV, United’s u21 stopper said:
“You’ve got to up your performance with the first-team so it helps you step up.
“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.”