Kewell, who played a few minutes for Galatasaray last week in his first competitive match since December, would miss the Turkish club's last two matches after re-injuring his groin, assistant coach Johan Neeskens was quoted as saying by the Sydney Morning Herald.
"He started again with the team a week ago but now, the second week, he gets some pain and he didn't feel comfortable," Neeskens said. "Tuesday, he stopped again and couldn't continue the work because it's too painful.
"I don't know if he really is to make the World Cup at this rate."
Neeskens described the injury as recurring.
"Stage by stage he got better but then he fell back again. He chose to stop practising and take treatment but sometimes after three or four days and he stops treatment and starts training again, it comes back again," he added.
"If he cannot participate, it's a big loss for Australia."
Kewell, who returned to Australia for groin surgery earlier this year, is likely to be named on Tuesday in coach Pim Verbeek's 30-man preliminary squad for the finals, regardless of his fitness.
The former Leeds United and Liverpool player has only two weeks to recover before Australia's friendly against New Zealand in Melbourne on May 24, and will have just two other friendlies against Denmark and the United States to prove his match fitness.
Australia are in Group D with Germany, Serbia and Ghana at the June 11-July 11 tournament.
