The 28-year-old came through United’s academy and the Red Devils are reportedly keen on signing the stopper should one of David De Gea or Dean Henderson leave this summer.
Ole Gunnar Solskjaer is expecting one of the duo to depart after switching between the pair throughout the season and the Norwegian knows the situation cannot go into a second campaign.
Johnstone had an impressive campaign with the Baggies, despite their relegation. He was recently named in England’s provisional squad for the European Championships.
However, the Sun claim West Brom have slapped a prohibitively-high £20m asking price on Johnstone in a bid to fend off interest in the goalkeeper.
The Baggies are determined to bounce straight back into the Premier League and Johnstone is considered crucial to the club’s hopes of emerging from the Championship.
Johnstone only has a year to run on his deal in the Midlands and would understandably be keen on a return to the Theatre of Dreams, where he failed to make a single senior appearance.
West Ham have also shown interest in Johnstone but the Hammers would be put off by West Brom’s asking price.
United are expected to sign Tom Heaton on a free transfer after the England international’s departure from Aston Villa.
Lee Grant and Sergio Romero will depart United this summer, meaning Solskjaer’s side lack a third goalkeeper in their ranks.