The 28-year-old is currently in his third season with the Baggies, and he has been in impressive form despite the club’s relegation fight.
He has made 120 saves over the course of the league season, and only Leeds United’s Illan Meslier has fared better (121), having played an additional game.
The Baggies are eight points adrift of safety with only eight matches left in the campaign, and they appear likely to go down next month.
With this in mind, Johnstone could be sold by the Midlands outfit, given he has yet to renew his contract, which expires in 14 months.
Tottenham Hotspur have recently been linked with a move for Johnstone, with the long-term future of club captain Hugo Lloris currently uncertain.
They could now face competition from the Red Devils for the shot-stopper, who could be available for as little as £10 million this summer.
The futures of Henderson and David de Gea have been a regular talking point, and it appears that the latter is the likelier to push for an exit.
Henderson has recently got the nod over the Spaniard for the league games, and reports have emerged that the club are open to offers for the 30-year-old.
It has also been suggested that the club could offer De Gea as a makeweight to sign long-term target Jan Oblak from Atletico Madrid.
However, it is well known that Atletico won’t sell their goalkeeper for anything less than the £103m buy-out clause, which United are unlikely to match.
Johnstone does not have the high-profile reputation of Oblak, but he has proved a quality keeper during his time in the Midlands.
A step up to an elite Premier League club will be the next challenge for him, and it remains to be seen whether United look to re-sign their graduate.
Johnstone was previously with the club between 2009 and 2018, but he did not make a single first-team appearance.
Stats from Transfermarkt.com