"When players have been out for a while, it takes a while to get back to your best with confidence, sharpness and zip," stated Wenger, whose team are third in the Premier League.
"That will come back with Theo."
Walcott, who cost the Gunners £5m when they signed him from Southampton in 2006, has eight full England caps and has scored three goals for the international team - all in a World Cup qualifier in Croatia in September 2008.
The winger has also featured for England's Under-21s, and Wenger said before last summer's European Under-21 Championship that he did not want him to play for both international sides because of the workload.
However, England Under-21 boss Stuart Pearce played down Wenger's fears, saying: "If you're picked for the national team play all the games you can."
When players have been out for a while, it takes a while to get back to your best
The Gunners manager also said a good season for Arsenal and their England internationals could boost Capello's chances of success at the tournament in South Africa, which starts on 11 June.
"They will be stronger at the World Cup if they come with a trophy, and it has been a good season, than without," he stated.
Meanwhile, Capello will name his squad at the weekend for England's friendly against Egypt at Wembley on 3 March.
He must pick an initial 35-man World Cup squad by 12 May, with a 1 June cut-off date set by Fifa for teams to name their final 23-man squads.
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