The Halifax-born Widdop was ruled out for around six weeks after damaging knee ligaments in St George Illawarra's NRL defeat to Sydney Roosters on Tuesday, less than 24 hours after being named in England coach Wayne Bennett's 20-man squad for the international against Samoa in Sydney on May 6.
Ratchford, 28, will now join his club captain Chris Hill in the squad that flies out to Sydney on Saturday, hoping to win his second cap.
Ratchford made his debut in England's international against France in Avignon in October but missed the entire Four Nations Series after damaging a knee in training.
He missed the first seven matches of the season for his club, including the World Club Series clash with Bennett's Brisbane Broncos, but has looked impressive since making his comeback at the end of March.
Widdop, who has been in good form for the Dragons so far this season, would have been an automatic selection against Samoa, lining up in the halves alongside either Castleford's Luke Gale or Wigan's George Williams.
Bennett will now go with Gale and Williams, with Ratchford providing cover for a host of positions.
"Gareth is one of the in-form players in the game at the moment so it's disappointing to lose him from the squad," said Bennett.
"He's been part of the national team since 2010 so I can imagine he'll be pretty frustrated not to be involved with the players and against Samoa.
"We wish Gareth all the best and hope to see him back out there performing to the high level that he has been doing this year."
Ratchford, who can cover most positions from full-back, hooker, loose forward, centre and half-back, will play for Warrington at Hull on Friday before flying out with the rest of the Super League-based players.
A Wolves spokesperson said: "Everyone at Warrington would like to congratulate Stef on his call-up, and we'll see him again in time for our Challenge Cup sixth-round tie against the Widnes Vikings."
