It is claimed that Villa have already held discussions with United regarding a January loan deal, but with no agreement reached as yet, a trio of Championship rivals have reportedly sensed an opportunity.
Cities Norwich and Bristol, who have each kept four league clean sheets all season, and Wigan Athletic, whose manager Warren Joyce worked with Johnstone at Old Trafford, are all said to have put forward their cases.
Villans boss Steve Bruce also has strong United ties, however, and a move to Villa Park will likely appeal to the shot-stopper, provided he is offered regular first team football.
With Mark Bunn and Jed Steer both sidelined, Johnstone would have only Pierluigi Gollini to compete with for Villa's number one spot.
And the Italian's inconsistent form since arriving from Verona in a £4.5 million deal this summer could give Johnstone the upper hand on his former United teammate.
Norwich have second-choice keeper John Ruddy out of contract next summer, while Bristol City have ditched Richard O'Donnell in favour of Frank Fielding in recent weeks.
Could the Canaries or Robins really beat Villa to Johnstone?