Johnstone supplanted the harmed Aaron Ramsdale of Arsenal however has himself currently pulled out with ailment.
Forster, 34, won the remainder of his six England covers in 2016.
Gareth Southgate's side host Switzerland on Saturday prior to taking on Ivory Coast on Tuesday with both well disposed games at Wembley.
Forster joins Southampton partners James Ward-Prowse and Kyle Walker-Peters in the crew.
Mitchell and Walker-Peters acquire first England call-ups
Britain crew
Goalkeepers: Jordan Pickford, Nick Pope, Fraser Forster
Protectors: Conor Coady, Marc Guehi, Harry Maguire, Tyrone Mings, Tyrick Mitchell, Luke Shaw, John Stones, Kyle Walker-Peters, Ben White
Midfielders: Jude Bellingham, Conor Gallagher, Jordan Henderson, Mason Mount, Declan Rice, James Ward-Prowse
Advances: Phil Foden, Jack Grealish, Harry Kane, Bukayo Saka, Emile Smith Rowe, Raheem Sterling, Ollie Watkins
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