Hugo Lloris' indifferent form for Jose Mourinho's team this season has led to the club searching for a potential replacement and Johnstone is regarded as a reliable, and inexpensive, option to challenge or replace the France No. 1, who has been deputised by ex-England keeper Joe Hart this season.
The former Manchester United youngster, who was loaned out to six different clubs during his nine-year career at Old Trafford, earned his first senior England call-up for the World Cup qualifiers against San Marino, Albania and Poland last month after emerging as one of the most consistent keepers in the Premier League at West Brom this season.
But with Sam Allardyce's team facing a battle to avoid relegation -- West Brom are eight points adrift of safety with eight games to play -- Johnstone's future at the club is now in doubt due to the financial impact of dropping back into the Championship following promotion last season.
Having arrived from United in a £6.5 million deal in 2018, sources have told ESPN that Johnstone could be sold for as little as £10m this summer due to the keeper having just one year left to run on his contract at The Hawthorns.
The prospect of signing a potential England keeper for such a low fee has also prompted interest from West Ham and Leeds.
Meanwhile, sources have confirmed that United stand to receive an undisclosed percentage of any transfer fee due to a sell-on clause inserted in the deal with West Brom in 2018.