Hargreaves, 28, has been out of action for almost 15 months after major surgery on both knees to cure a tendinitis problem - but he is almost ready to return to full training.
United boss Sir Alex Ferguson recently revealed his 2007 signing from Bayern Munich is working well with physios and needs to regain the confidence needed before being ready for competitive football.
Capello, who is refusing to rule Hargreaves out of his plans, said: "Why not? He can because he has time to recover everything. He has to play. Players who do not play will not be with me."
The Italian added: "I spoke to him two months ago, I met him in Manchester and he was just ready to start training. But now he is not fit. He has six months to recover his good form but he needs to play. I will choose players that will be fit in May, the players that play all the games."
Hargreaves was England's player of the tournament for the 2006 FIFA World Cup in Germany but has had serious injury problems since - his last game for his country was against USA in the season before last.
Since then Gareth Barry has established himself as Capello's first-choice defensive midfielder, although Capello likes players such as Hargreaves who can play in more than one position.
The England boss is "more or less" settled on his squad for South Africa and is hoping for a clean bill of health come the end of the season.
He said: "More or less, I know all the players. I follow the players and check on them but we have to wait. 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."
England will be staying in Rustenburg, after Capello's fears over the state of the training pitches were eased.
"The facilities are OK, there were some problems with the pitches but I spoke to people there and they will be okay for June when we are in South Africa," he said. "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 - everything is OK."
Before that, Capello wants to play Egypt at Wembley on 3 March, then arrange a friendly with Mexico.
He added: "We have to decide. We are speaking with these countries but have not decided yet. "I think we will play 3 March at Wembley, against Egypt, then we have to find the date to play against Mexico. There are some problems, I don't know if we will play Sunday or Monday.
"All the friendly games we played were against European teams and we know the style of these teams. We have to play against South Americans and African teams with a different style."