Manchester City boss Roberto Mancini had already ruled Barry out of Sunday's trip to West Ham after the player limped out of Wednesday's defeat to Tottenham at Eastlands.
And Mancini confirmed the damage from a sprained ankle will keep Barry on the sidelines for a month, placing a major question mark over his chances of making the World Cup in South Africa.
England coach Capello is due to name his World Cup squad on June 1 and Barry's chances of being called up now look in doubt.
"I feel very sorry for him," said Mancini. "Gareth is a fantastic guy and a fantastic player.
"I hope he can play in the World Cup but Fabio Capello and his medical staff must decide."
Barry does have time to recover as a four-week absence from today would still give him seven days to get fit ahead of England's opening Group C encounter with the United States in Rustenburg on June 12.
However, the former Aston Villa man will definitely be absent from the friendlies against Mexico and Japan, which cover two training camps in Austria, time Capello views as essential preparation ahead of departure to South Africa.
Given Barry's status as a regular member of Capello's first-choice line-up and the fact alternatives are not extensive, with widely-accepted replacement Michael Carrick struggling for form at Manchester United, the chances are Capello will include Barry in his 30-man provisional squad that will be announced on Tuesday.
He will then need to assess the player's recovery before finalising his 23-man party on June 1.
"I don't know exactly how bad, maybe three to four weeks, so I hope he can still go," said Mancini.
"He deserves to play in the World Cup."