Johnstone, 25, has been outstanding for Villa since joining on loan last summer. The 17 clean sheets he has earned have dramatically improved the Birmingham-based club's chances of making a return to the top-flight.
Steve Bruce's side are currently fourth and have all but guaranteed a spot in the play-offs, with automatic promotion still mathematically possible.
Coton, who coached Johnstone during his decade long stint as United's goalkeeper coach, praised the Englishman for his performances and expressed hope that Johnstone would extend his stay in the Midlands.
"He has been tremendous," Coton told Tribalfootball. "I've known him since he was 12 years of age, I've previously coached him in his younger days before I retired.
"I know what he's like physically and mentally. He's been instrumental in the club rising up the Championship.
"He's made a couple of mistakes but every player does. He's always bounced back, took it on the chin and learnt from it, and certainly if we go up then we'd like to have Johnstone on board.
"If we don't then Man United and Sam have a decision to make on what he does."
Last month, United triggered a one-year extension to Johnstone's contract, which will keep him at the club until 2019.
But with David De Gea, Sergio Romero and Joel Pereira ahead on the pecking order at Old Trafford, it is likely the club activated the clause in order to secure a future transfer fee.
Speaking in early March, Johnstone revealed he had yet to hold talks with United, but insisted he was enjoying his experience at Villa Park.
He told Villa's website: "I enjoy it here. I love playing football here and training every day.
"I want to keep playing football. I'm enjoying the number of games I'm playing."
Despite being included in the United first-team squad in 2011 at 18 years of age, Johnstone has yet to make a single appearance for the club.
Johnstone has since spent time on loan at seven different clubs and Coton believes this exposure to senior football has helped the former England U20 international gain invaluable experience.
"People say to me that you're the best keeper when you're 30, but I don't agree with that. He has first-team experience now and when you get to 25 that's when you're peaking physically. You've had enough experiences behind you.
"I don't know how many games he's had now but its certainly over 100 first-team games. He'll be peaking at the right time now so hopefully we can get to the Premier League and he'll be coming along with us."
You can read more about Tony Coton's incredible life and career in his biography, There To Be Shot At, which can be bought by clicking on this link - decoubertin.co.uk/Coton.
Follow Andrew Maclean on Twitter: @andrewmaclean30