Okada told to call up European-based players
Japan Football Association (JFA) president Motoaki Inukai said on Monday that Okada must build on last week's solid 2-0 win over Bahrain in preparation for this year's World Cup.
"I have told him not to hesitate (calling up the European-based players)," Inukai told reporters as speculation mounted on who Okada would pick for South Africa.
"He has to put out the team he wants. It's a World Cup year so we should have more room to negotiate."
The Blue Samurai take on Serbia in Osaka on April 7.
Victory over Bahrain with a full-strength side in a dead-rubber Asian Cup qualifier eased the pressure on Okada following Japan's third-place finish at last month's East Asian championship in Tokyo.
Japan face World Cup-bound South Korea at home on May 24 before flying to Europe for warm-up games against England in Austria on May 30 and Ivory Coast in Switzerland on June 4.
Okada's side have been drawn alongside Cameroon, the Netherlands and Denmark in Group E at the World Cup.