King was called up to the latest England squad after some commanding displays for his club Tottenham Hotspur.
However, the 28-year-old suffers with a long-standing knee problem that restricts him to just one match every seven days.
And on Tuesday it was confirmed that King would return to White Hart Lane, having been assessed by the England medical team.
OptimisticIt has been suggested that King's fitness issue means his England future is bleak, and Capello, who rates the defender highly, remains unsure if he could still have a role to play.
He said on Sky Sports News: "We hope (he has an international future), because he's a very interesting player - one of the best centre-backs. But I don't know the future.
"We've followed Ledley King a long time. The last four games he played all the games. For this reason we chose him.
"He came here. The national team's medical staff checked him. There was some problem with the knee. The day after we found the same problem with it, so we decided Ledley would return to Tottenham.
"He's a good player. I watched the game. He always plays well. When I choose I try to choose the best players.
"When the medical staff checked on the situation we decided that he could go back."
RedknappSpurs boss Harry Redknapp branded King's call-up as "mad" and "pointless", sparking reports of a club versus country row.
When asked of his relationship with Redknapp, Capello replied: "It's a normal relationship like with the other managers of the Premier League.
"I don't understand why he spoke about King and not about (Alan) Hutton - the Scottish manager included him in his squad after he didn't play for two months.
"It's the same thing. I don't know why he was happy about Scotland and not happy with us."