"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 Cup winners Liverpool kick off the season with the Community Shield against champions Chelsea in Cardiff on Aug. 13 before their first premier league match at promoted Sheffield United on Aug. 19.
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.
