Johnstone is currently on loan at Villa Park, and following some impressive performances, Steve Bruce is keen to strike a permanent deal with his former club.
The 24-year-old originally joined Villa back in January and United were happy to let him go on another season-long loan deal when Bruce made another approach in the summer.
Now the Championship club want him permanently and it remains to be seen whether Jose Mourinho will sanction the move or decide to give him a chance at Old Trafford.
Either way, Bruce is ready to pursue it.
"It would be great to keep him here for the next few years as Villa's number one," Bruce revealed. "I will need to ask the question. The ideal move for us would be to buy him in January.
"You've seen it, a young goalkeeper coming in and getting better with every game he plays. The difference in him since he's started in January to now is light years. He's confident, playing every week and he's playing real football."