Since taking over from Steve McClaren 17 months ago Capello has rebuilt England's shattered confidence and overseen an impressive qualifying campaign.
Now, with the Premier League season over and just next Saturday's FA Cup final between Chelsea and Portsmouth to go, all eyes will be on him when he announces his squad at Wembley.
"For me the World Cup will start tomorrow when the players know the names of who will go to (training camp in) Austria," Capello told Reuters in an interview on Monday.
"It's not so easy because 30 is very important number but 23 is more important and telling seven they have to go home will not be easy for me."
"We do have problems and I have 24 hours to decide but I will ensure that the players will be fit," the Italian said.
That rules him out of the May 24 friendly against Mexico and the game against Japan at England's high altitude training camp in Austria.
Wayne Rooney, who limped out of Manchester United's match against Stoke City on Sunday with a reoccurrence of a groin problem, is less of a worry, though Capello will want his main striker fully rested after a flat end when he failed to score in his last four games.
King's powerful, athletic displays in central defence have made him a late shot to make the squad, despite the fact that his long-term knee problems severely limit his training and usually prevent him playing two games in a week.
King sits on top of the "Capello Index," a new system launched by the England manager on Monday that measures player performance based on a combination of 500 different possible individual contributions to a match situation but the coach said he was less impressed by how the defender had fared under his own new product than on the pitch.
"The Index is good as it helps the fans understand but I am more happy that Ledley King has played four games in 15 days and happy that he played four games well," he said.
"No, that deals with performance but it is important to pick players with the right mentality," Capello said. "You have to have players who are happy on the pitch, happy on the bench and happy to be in the stand - this is very, very important."
