Johnstone is eager to step in with David De Gea and Anders Lindegaard likely to be rested.
"We are all working hard every day to try and get in the team but I think League Cup week is a chance to prove yourself if you get picked," 19-year-old keeper Johnstone told the Manchester Evening News.
"At a club like this they want to win as many trophies as possible so it just depends. It is a fine balance between making sure you win the game and possibly giving some young lads a chance. It is well known the manager here will give you a chance if you are good enough and then it is up to you to take it.
"These are lads who are just a few years older than us so they are a recent incentive for us.
"It is a big season for me. I am third choice now so the chance could come quickly, so you always have to be ready."