Mortlock has won 60 caps for his countryAustralian skipper Stirling Mortlock has been forced to sit out training to give the Wallabies a major injury scare ahead of the World Cup.
The 30-year-old centre suffered what he described a recurrence of a disc problem but is confident of returning to training soon.
"It is definitely a concern because he is such an influential player," said Australian coach John Connolly.
"But the doctor thinks it will settle down as quickly as it came."
The Wallabies open up their World Cup campaign against Japan in Lyon on 8 September.
Meanwhile, Australia's management team say that Adam Ashley-Cooper (groin) and Guy Shepherdson (knee) are on track with their recoveries.
Fly-half Berrick Barnes has been given the all-clear to resume training after recovering from a niggling groin injury that has sidelined him for the past three weeks.