His condition, patellar tendinitis, can be described as inflammation of the tendon that attaches the quadriceps (thigh muscle) to the tibia (shin bone).
The Canadian-born midfielder had been expected to rejoin the United squad in August, but was forced to delay his comeback and has still not returned to full training with the rest of the team.
He won the last of his 42 international caps against USA at Wembley in May 2008, but Capello says if he starts playing regularly for his club, he could still be picked in the 23-man squad for South Africa.
"Owen has six months to recover his good form, but he needs to play football," added Capello. "I will choose players that will be fit in May, the players that play all the games.
"When I decide the 23 players that will stay with me, I hope the best players will be fit. This is the reason why I have to follow more players."
Meanwhile, Capello has confirmed that England's training base during the World Cup finals will be in Rustenburg at the Royal Bafokeng sports campus.
"The facilities are OK, there were some problems with the pitches but I spoke to people there and they will be OK for June when we are in South Africa," said Capello.
"We found Rustenburg was the best place for us. It is quiet, good for security, is close to the stadium where we play the first game."
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