The 28-year-old has been in outstanding form for the Baggies this season with his performances earning him a call-up into the England squad last month.
Johnstone is out of contract at The Hawthorns at the end of next season.
And his contract status – along with his impressive performances – mean he is likely to be interesting a host of Premier League clubs this summer if Albion do go down.
According to reports, Albion will demand in the region of £10million for the shot-stopper they signed for £6.5million in July 2018.
Johnstone, who began his career with Manchester United, is understood to be very happy at The Hawthorns.
But like all Albion players, he will see his salary cut – by either 40 or 50 per cent – if they are relegated.
He also knows his chances of staying in Gareth Southgate’s England plans will be reduce if he is playing Championship football next season.
And that means – while Johnstone is in no rush to leave and Albion don’t really want to sell – the club wouldn’t feel it was right to stand in his way if a suitable offer does arise this summer.
A host of clubs in the Premier League have been linked with a move for Johnstone in recent months.
Leeds – who currently have 21-year-old Illan Meslier as their number one – are believed to be interested.
Lukasz Fabianski at West Ham turns 36 later this month – with the Hammers potentially in the market for a new glovesman at the end of the current campaign.
Tottenham have also been linked with a move for Johnstone – with Hugo Lloris potentially set for a reunion with Mauricio Pochettino at PSG.
While Albion fans will be disappointed to see Johnstone leave if he does move on, the club is very well stocked in the goalkeeping department.
Current number two David Button has made more than 200 professional appearances and at 32 has plenty of experience if the Baggies do find themselves in the Championship next season.
Meanwhile, Alex Palmer will return to the club in the close season having enjoyed two excellent loan spells over the last two years.
The 24-year-old kept more clean sheets than any other goalkeeper while on loan in League Two with Plymouth during the 2019/20 season.
And this season he has impressed at Lincoln as Michael Appleton’s side look to win promotion to the Championship.
The Baggies also have the hugely promising Josh Griffiths on their books.
The 19-year-old has played 37 times for Cheltenham this season.
And his impressive performances at Whaddon Road led to him being called into the England under-21 squad last month.