Kangaroos enforcer Michael Crocker's World Cup campaign is being threatened by a broken rib.

While the Melbourne Storm backrower has not been ruled out of World Cup duty, his painful injury, suffered in the Storm's record 40-0 grand final loss to Manly has cast serious doubts about his involvement in the 10-team international tournament which kicks off in 17 days.

"We initially thought it was rib cartilage damage but x-rays have revealed a cracked lower rib," said Storm physiotherapist Mary Toomey.

"You wouldn't think he would be able to do too much for a few weeks."

The ARL's medical staff and coach Ricky Stuart were notified of the extent of Crocker's injury on Wednesday but Stuart couldn't be reached for comment.

But it's understood the UK-bound 28-year-old could be given more time before his injury is re-assessed.

It could be that Stuart is prepared to let Crocker miss Australia's World Cup opener against New Zealand at the Sydney Football Stadium on October 26 and have him back for the clash with Great Britain at Melbourne's Telstra Dome on November 2.

If Crocker was ruled out, Brisbane's Sam Thaiday or Roosters hardman Nate Myles could score a call up.

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