"Harry is currently undergoing treatment in hospital following a surgical procedure," the club said on their Web site after reports that the forward had been suffering from gout.
"He is expected to make a complete recovery and should be able to resume training in approximately six weeks' time."
FA
Kewell was man of the match in Australia's 2-2 draw with Croatia in Germany last month that took his team into the second round of the World Cup for the first time.
He then missed Australia's subsequent defeat to Italy. Team mates said he had been suffering from gout.