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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The denial comes after the Mail on Sunday reported that the former British prime minister met the president's son-in-law and key adviser, Jared Kushner, at the White House last week.
The paper claims the meetings were to discuss working for Mr Trump.
Mr Blair's office said the story was an "invention" and that he continues to work for peace "in a private capacity".
After he left Downing Street in 2007, Mr Blair took the role of Middle East envoy for the Quartet Group, made up of the European Union, US, Russia and United Nations.
His spokesman said: "Mr Blair has made no such 'pitch' to be the president's Middle East envoy.
"Neither has he had any discussions about taking such a role or any role working for the new president. He has been working on the peace process for 10 years.
"He continues to do so. He does so in a private capacity. He will continue to do it in that way. Period."
Mr Blair launched a campaign last month to try to persuade the British people to change their minds about leaving the European Union, following the Brexit referendum held last year.

Unite said 1,000 of its members would walk out from 02:00 BST next Tuesday.
The union accused First of taking an "extremely aggressive" stance by sacking a branch secretary and two other workers for "legitimate trade union activity".
First Leeds said the "unjustified action" would affect tens of thousands of people across the city.
Drivers have already held two 24-hour strikes, with Unite claiming they earn Â£2 less an hour than other First drivers in the region.
The firm dismissed this as "nonsense" and said the union had failed to back up "this ridiculous suggestion".
First Leeds also disputed the "aggressive" stance alleged by Unite and said it was investigating some very serious allegations.
No members of staff have been disciplined for taking part in industrial action or for any legitimate trade union activity, it said.
First Leeds offered a 38p per hour rise frozen until May 2018, which Unite described as "insulting".
Unite regional officer Phil Bown said: "The management needs to stop ducking and diving and commit to holding genuine and meaningful talks, otherwise this strike could go on for some time with an adverse impact for bus users in Leeds - something we have desperately sought to avoid."
Paul Matthews, managing director of First Leeds, said: "I'm very disappointed and deeply frustrated that Unite plan to cause such huge disruption for our customers in Leeds.
"This is not a last resort as Unite claim - not only do we have a planned meeting before Tuesday to try and resolve the dispute, but Unite have repeatedly refused to let all staff vote on our above inflation offers."
The bus provider plans to provide a reduced service in Leeds, with buses in Bradford, Huddersfield and Halifax all unaffected.
Injured Southampton goalkeeper Fraser Forster has been replaced by club-mate Alex McCarthy for England's World Cup qualifier in Slovakia on Sunday.
[NEXT_CONCEPT]
Tony Blair has not pitched to become a Middle East envoy for US President Donald Trump, a spokesman has said.
[NEXT_CONCEPT]
Bus drivers and other workers for First Leeds are to stage an indefinite strike in a worsening row over pay.