The goalkeeper, who joined the club from Manchester United in 2018 after numerous loan spells in the EFL, has been one of the Baggies' key players over the last few seasons.
He has played 155 times over that four-year period, keeping 41 clean sheets, helping them to Premier League promotion in 2020.
However, he has now fallen out with boss Ismael, who joined the club from Barnsley at the start of the season, who said there was an 'internal issue' with his player.
"We had an issue internally," said Ismael, after their 2-0 defeat on Wednesday.
"It is solved now but that is the reason he wasn't in the squad and won't be on Saturday, but after that, he will be back."
Johnstone's contract expires in the summer, meaning he can leave the club on a free transfer.
The likes of West Ham, Southampton, and Newcastle have all been linked with the 'keeper, who has made three England caps.
Despite Ismael saying he will be back in the squad after this weekend, the club may decide to cash in on a potentially unhappy player before the transfer window closes on Monday.
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