Johnstone has been sent home from the England camp as they prepare to face Belgium in an international friendly on Tuesday, after picking up an elbow injury at the weekend.
With Palace likely to remain in the Premier League next season after improved results under Oliver Glasner, Johnstone would have been looking to maintain his place as a backup with the Three Lions.
The 31-year-old won back his place in the Eagles' side under Glasner after dropping to bench in the final few games of Roy Hodgson's reign at the club.
In the four games after Johnstone regained his place, Palace took five points, and also kept a clean sheet against Burnley at Selhurst Park.
However, the Evening Standard claim that Johnstone could miss the rest of the season, jeopardising his chances of being part of Gareth Southgate's squad in Germany.
Jordan Pickford is the undisputed number one, but Johnstone has become the third-choice goalkeeper of late thanks to his performances with Palace.
Aaron Ramsdale remains second-choice, but there is now a possible battle for the final slot in Southgate's squad.
James Trafford has been called up to the current squad despite being dropped by Burnley last week, while Jack Butland is also having a fine season with title-chasing Rangers in Scotland.
At Selhurst Park, Johnstone is set to be replaced by Dean Henderson, who himself may have ambitions of working his way back into the squad after his injury nightmares of the last two years.
Should Johnstone not feature much until the end of the season, Southgate may be wary of a lack of match sharpness, in case the unlikely scenario occurs
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