With David de Gea looking increasingly likely to end his stint at Old Trafford this summer, Ole Gunnar Solskjaer is expected to pursue a new shot-stopper ahead of the 2021-22 campaign.
Jan Oblak, Hugo Lloris and Mike Maignan have all been linked with a move to United, while a recent report claimed that former United goalkeeper Tom Heaton is another option for the Red Devils.
Now, ESPN reports that another ex-academy starlet in Johnstone is on Solskjaer's shortlist, with the 28-year-old impressing for West Brom as they stare down the barrel of relegation.
However, the report adds that West Ham are currently leading the race to sign the Baggies goalkeeper, as David Moyes can offer him regular first-team football, while he would be expected to play second fiddle to Dean Henderson at Man United.
Johnstone failed to make a single appearance for Man United before leaving permanently for West Brom in 2018, where he has kept six clean sheets in 33 Premier League games this term.