Johnstone is currently on his fifth loan spell away from Old Trafford, turning out for Doncaster Rovers for the second consecutive season.
Having moved to the Keepmoat Stadium in January 2014 - remaining until the close of last term - the stopper rejoined the club in October, signing on a short-term deal until January 2015.
Spells in previous years with Scunthorpe, Yeovil and Walsall have seen the youngster progress at a rapid rate, and he is highly rated in Manchester - appearing for the club during their pre-season International Champions Cup tour.
The 21-year-old signed a new three-and-a-half year contract with the Red Devils earlier this year, and he has no reservations about dropping down to League One in the search for consistent football - having made his second debut for the promotion-chasing club in Saturday’s 2-1 victory at Bradford.
With opportunities likely to come at the Keepmoat, Johnstone admitted that he is still keen on making the number one jersey his own at Old Trafford, having joined the club at academy level.
Speaking to the Doncaster Free Press, the former England Under-20 international said: “Old Trafford is obviously where I want to be.
“It is difficult. Obviously [David] De Gea is not much older than me.
“I’ve been there since I was ten years old and sometimes you have to come away and learn your trade, to go back and push.
“It’s every boy’s dream to play for Manchester United and I signed a new contract not long ago.”
With Doncaster keen to get their promotion race back on track after a tough start to the season, Johnstone will be looking to prove himself to the onlooking Red Devils management, with a place at Old Trafford his for the taking.