Uncapped McCarthy, 26, was called up after Forster, 28, returned to his club for treatment on an arm injury.
The ex-Crystal Palace keeper has played three times for England's Under 21s.
Joe Hart, on loan at Torino from Manchester City, and Burnley's Tom Heaton are the other goalkeepers named in Sam Allardyce's first England squad.
McCarthy joined Saints in the summer as deputy to Forster but has yet to feature in a game.
Goalkeepers: Joe Hart (Manchester City), Tom Heaton (Burnley), Alex McCarthy (Southampton).
Defenders: Gary Cahill (Chelsea), Nathaniel Clyne (Liverpool), Phil Jagielka (Everton), Danny Rose (Tottenham Hotspur), Luke Shaw (Manchester United), Chris Smalling (Manchester United), John Stones (Manchester City), Kyle Walker (Tottenham Hotspur).
Midfielders: Dele Alli (Tottenham Hotspur), Michail Antonio (West Ham United), Eric Dier (Tottenham Hotspur), Danny Drinkwater (Leicester City), Jordan Henderson (Liverpool), Adam Lallana (Liverpool), Wayne Rooney (Manchester United), Raheem Sterling (Manchester City), Theo Walcott (Arsenal).
Strikers: Harry Kane (Tottenham Hotspur), Daniel Sturridge (Liverpool), Jamie Vardy (Leicester City).
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Injured Southampton goalkeeper Fraser Forster has been replaced by club-mate Alex McCarthy for England's World Cup qualifier in Slovakia on Sunday.