The goalkeeper, who has been in good form for the Villans in the Championship, was set to leave Old Trafford on a free transfer in the summer as his contract ran down - a factor that had led Villa to make attempts to sign him for a cut-priced fee.
However, the Daily Mail has revealed that United have opted to activate a clause in Johnstone's deal at Old Trafford to automatically extend his contract by a year and tie him down to the club until June 2019.
Of course, Villa would still be able to head to the negotiating table with United to try and convince them to part with the shot stopper on a permanent basis.
Doing so would come at a cost now though, and it remains to be seen whether Villa manager Steve Bruce can persuade his board to hand over whatever the Red Devils wants for Johnstone's signature.
Johnstone joined Villa on a season-long loan in July and has featured in every single one of Villa's 20 league games to date - keeping eight clean sheets in the process.
Always inevitable that Man United were going to extend Sam Johnstone’s contract. Should’ve been expected. If it does materialise, I just hope we wrap up a permanent fee ASAP. #AVFC— Matt Lynch (@mattlyynch) December 5, 2017
Such has been his good form for the Midlands outfit, other Premier League clubs are thought to be sniffing around with the possibility of making firm offers for the 24-year-old.
A bidding war would be something Villa would be keen to avoid due to their position in English football's hierarchy, but Bruce could be able to convince Johnstone to forego top flight football with regular game time at Villa Park and with the chance to continue his development in an environment he has enjoyed so far this term.
Johnstone has risen through the youth ranks with United and, whilst he is yet to make his senior bow for them, has also spent time out on loan at five other lower league sides including Doncaster Rovers, Scunthorpe United and Preston North End.