Mortlock explained the injury was a recurrence of a disc problem he suffered two seasons ago and he was confident of being back on the training paddock on the weekend.
However, Wallabies head coach John Connolly admitted the site of Mortlock moving gingerly had given him a scare.
"It is definitely a concern because he is such an influential player," said Connolly. "But the doctor thinks it will settle down as quickly as it came."
In other injury news, Adam Ashley-Cooper (groin) and Guy Shepherdson (knee) are back on track with their training while rookie fly-half Berrick Barnes has also been given the all clear to resume training after recovering from a niggling groin injury that has sidelined him for the past three weeks.
Barnes had his first run with ball in hand since a training camp in Sydney prior to the team's departure from Australia on August 23.
Meanwhile, Australia full-back Chris Latham has spoken of his relief to be part of Australia's World Cup squad after making a better than expected recovery from a knee reconstruction this year.
"It is a bit hard but there is a sense of relief," said Latham in Paris.
"There were times when I didn't think I would be - so it is very exciting."
However, the 31-year-old custodian said his inclusion in the Wallabies' World Cup squad was only the first part of his goal of helping Australia to claim an unprecedented third World Cup.
"There is a little bit of a relief I am here in Paris but obviously there is a job at hand to be done yet and the full completion of the long-term goal," he said.
"So obviously I just can't wait for it to start and to just get into the games."