Capello, who must cut his 30-man provisional squad for South Africa to 23 by Tuesday, has made changes to the side that beat Mexico 3-1 at Wembley on Monday and given starts to Bent and Tottenham Hotspur midfielder Tom Huddlestone.
"Bent is fit, he will play tomorrow from the start," Capello told Sky television ahead of the match in Graz, Austria, on Sunday.
Bent is competing for a place in the final squad with fellow strikers Wayne Rooney, Peter Crouch, Emile Heskey and Jermain Defoe.
The Sunderland striker, who missed the game against Mexico with a hamstring injury, had been tipped by English media as the striker most likely to miss out on a place in the final squad.
"I already know the players who have been with us during qualification very well," Capello told reporters on Saturday.
"Here we have new players. For me these are the ones who it is most important to check.
"We need to know everything about what positions they take up on the pitch and what they do in key moments throughout the game."
England are still anxious about the fitness of holding midfielder Gareth Barry who is racing against time to be fit for the tournament after injuring ankle ligaments playing for Manchester City against Tottenham on May 5.
"He has improved a lot," said Capello. "He is getting better and better. The doctor says he is really good. But we have to wait until the last check next Tuesday. After that we will decide whether he will be with us in South Africa."
The battle to provide cover for the injured midfielder is between Huddlestone, Manchester United's Michael Carrick and West Ham United's Scott Parker.
While Carrick played against Mexico, Huddlestone's selection in the heart of the midfield for Sunday's game could spell the end of Parker's hopes.
"I want to see Huddlestone," added Capello. "The midfield is very important for the balance of the team."
Capello said Liverpool midfielder Steven Gerrard, who had sparked fitness worries after the victory over Mexico, should feature at some point against Japan.
"He is fit. He was tired after the game against Mexico," said Capello. "He won't play the first half against Japan but probably the second half."
England face the United States, Algeria and Slovenia in Group C at the finals, which start on June 11.
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