Okada said he planned to name 20 to 23 players for the friendly in Osaka on April 7 but did not know how many Europe-based players would be available.
"I think we will probably have to make do with a squad consisting of Japan-based players," Okada told reporters on Thursday. "It's possible the others could come. It's up to the JFA (Japan Football Association)."
JFA chief Motoaki Inukai this week insisted Okada called up his overseas players after last week's 2-0 victory over Bahrain to boost preparation for this year's World Cup.
Victory over Bahrain with a full-strength side in a dead-rubber Asian Cup qualifier went some way to easing mounting pressure on Okada.
Japan were booed off the pitch after their third-place finish at last month's East Asian championship in Tokyo.
They 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.
The Blue Samurai, who have never won a game at a World Cup tournament on foreign soil, have been drawn against Cameroon, the Netherlands and Denmark in Group E in South Africa.
