"Last year it was Walcott, he was very important for us in the World Cup qualifiers," the Italian told Gol Television in an interview screened in Spain on Monday.
"This year the player who has most improved has been Milner. He played three or four games with us and forms part of the national team.
"The other who has been crucial is Rooney because this year he has scored a lot of goals, and he has done so for us as well. He has been very impressive. I hope he maintains this form for South Africa."
The Manchester United striker has been in a rich vein of form for his club this season.
"There are three players who can make a difference at present," Capello added. "One is (Cristiano) Ronaldo, the other (Lionel) Messi, and Rooney.
"After these I believe there are two more. One is Kun (Sergio) Aguero and the other is Pato. They are very young and the future is for them."
With two months to go before Capello has to name his final squad for the World Cup finals, he said former captain David Beckham's Achilles injury had been a blow to his plans.
In recognition of his experience and leadership qualities, however, Capello said he had offered Beckham the chance to travel with the squad.
"He is an important player for us because he is soemone who is always ready to play, be it 10 or 15 minutes or half an hour. He is very important in the dressing room," Capello said.
"For the English he is a symbol. We have asked him if he wants to come with us to South Africa. It's up to him and how his tendon is recovering."
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