Boss Steve Bruce met United counterpart Jose Mourinho last week to discuss the 24-year-old, who spent the second half of the campaign on loan at Villa Park.
Another temporary deal now looks increasingly likely with Bruce, who is operating on a restricted budget this summer, eager to have a dependable No.1 in place sooner rather than later.
Johnstone was the third goalkeeper to play for the club during the course of the season.
Pierluigi Gollini, signed by Bruce’s predecessor Roberto Di Matteo, was moved out on loan to Atalanta in January just six months after joining in a £4million deal. Mark Bunn, meanwhile, showed inconsistent form in his seven appearances.
Johnstone was brought straight into the team after joining from United in the first week of January and went on to play 22 consecutive games, keeping eight clean sheets.
Veteran defender Chris Samba, who spent the final months of the season training with Villa, has meanwhile emerged as a possible target for Blues.
The 33-year-old former Blackburn and QPR centre-back first joined up with Bruce’s men in February following his release by Greek outfit Panathinaikos.
Bruce has left the door open for him to return in pre-season but reports now claim new Blues boss Harry Redknapp – who signed Samba for £12.5million when in charge at QPR – may offer his former charge a deal at St Andrew’s.