The 18-year-old has joined Scunthorpe on a one-month loan.
Johnstone told the club’s official website: “I spoke to Sir Alex Ferguson and he said ‘go and enjoy it, it will be a good experience for you’.
“It is a good amount of time for me to get games and get in the swing of things and become comfortable with people so hopefully I can impress.
“I feel my distribution is good, I command my box well, and I’ve got good hands and decision making. We’ll see how it goes.
“The manager Alan Knill has said settle in well, do your best, come and enjoy it, and the more games I get the better it is for me which is true.
“Hopefully I will get quite a lot of games and gain experience.”
