Wallabies No.8 David Lyons has been ruled out of the rest of the Rugby World Cup with a broken leg.

X-rays in hospital have confirmed Lyons fractured his fibula in falling awkwardly in a tackle during the second half of Saturday's 37-6 pool B win over Canada.

In his first start in the tournament, the 117-kilogram powerhouse was in damaging form before the 45th-minute blow at Bordeaux's Stade Chaban-Delmas.

The Wallabies will call for a replacement on Sunday or Monday but are assessing their options.

Brumbies back-rower Jone Tawake looms as the favourite as he is the player who best fits the power mould of Lyons and fellow No.8 Wycliff Palu.

Tawake was one of three, including lock James Horwill and veteran Matt Cockbain, who were on stand-by for Lyons and Palu when they were under injury clouds before the tournament.

Western Force No.8 Scott Fava would also be in contention.

But the selectors could also decide to choose a player from a different position due to a number of injuries in the backline.

The unavailability of Stephen Larkham and Scott Staniforth have left them short of options in the backs.

It was Lyons's first Test match since coming off the bench against South Africa in Sydney on July 7.

He was ruled out of the Tri Nations decider against the All Blacks with a blood clot and has been on blood-thinning medication since.

Lyons, who must give himself three injections a day, came off the medication for Saturday's match and enjoyed a destructive first half in drizzling rain.

"He was playing very well," coach John Connolly lamented. "He was playing very forcefully."

AAP