Kewell missed Australia's 1-0 win over European champions Greece in Melbourne on Thursday to remain in England for treatment on his injured groin.
The Liverpool striker will also miss next week's friendly against Holland and possibly the final warm-up against Liechtenstein but expects to be ready for Australia's opening World Cup Group F match against Japan on June 12.
"The injury is not ideal but I am expecting to be back and ready in time for the tournament," Kewell said.
"I'm so disappointed that I couldn't get back to Melbourne for the game against Greece but it was out of my control.
"I now have to focus on what is within my control, and that's working hard."
Australia play defending champions Brazil in their second pool match before tackling Croatia in what is looming as one of the toughest groups of the tournament.
"People might say that the Brazil game is the biggest but this is a World Cup where all three group games are important," Kewell said.
"To me, Japan and Croatia are just as big. We need to win, or at least not lose.
"It's great being in a group where all the matches have something at stake and where each team, if they have a good day, can win.
"Our first aim is to beat Japan, then a result against Brazil would be good, and then beat Croatia."
