Forster, 28, was expected to start Sunday's game in Trnava but was ruled out of Sam Allardyce's first game as manager after injuring his arm in training.
Little-known McCarthy, 26, who joined Southampton from Championship club Reading last month, was previously called up for friendlies against Ireland and Brazil in May 2013 but has hardly any experience of top-notch football.
Joe Hart, who was left out of Manchester City's opening three Premier League games of the season before moving on loan to Torino this week, is now likely to start for England on Sunday.
McCarthy has not featured for his club this season and has not played more than 10 league games in a single campaign since a loan spell with League One Yeovil Town in 2009-10.
His inclusion highlights how few England keepers are now playing in the Premier League.
With Forster and Stoke City's Jack Butland out injured, Burnley's Tom Heaton is the number two to Hart.
Heaton made his international debut in a friendly against Australia on May 27 and has never represented England in a competitive fixture.
(Reporting by Ian Rodricks and Ed Dove; Editing by Tony Jimenez)
