Kewell has been receiving treatment at Melwood after picking up a groin injury in last week's FA Cup final. Australia boss Guus Hiddink admits he is concerned over Kewell's fitness.
"I'm a little bit concerned about how his development will be," said Hiddink.
"He can be fit just before the starting game [against Japan].
"But also what we are doing here is not only building up the physical condition, but working on the tactics, the strategies towards the opening game.
"When a player is not available it has a limitation, but overall I hope he will recover."