Powerful lock and key line-out jumper David Giffin is in doubt for the Wallabies' sudden-death Rugby World Cup quarter-final against Scotland in Brisbane after aggravating a quadriceps muscle.

It is understood Giffin, who missed the game against Romania after sustaining a shoulder injury in the tournament opener against Argentina, will struggle to be fit for the Saturday night match.

The vice captain will be assessed again around midday.

If Giffin isn't fit, Justin Harrison will be promoted from the bench to the starting side with Daniel Vickerman taking his spot among the reserves.

The loss of one of Australia's most experienced forwards would come as a major blow as the Wallabies take on a strong, if underperforming, Scottish pack.

With a semi-final berth against South Africa or New Zealand on the line, Wallabies coach Eddie Jones said "last resort rugby," would be played by both sides.

"We're both out there to win the game ... we're going to get stuck," Jones said.

"We've got to play as though it's our last game because it could conceivably be.

"We want to go out there and absolutely put our best performance on the field, we want to be physical early on and set the platform for the game."

The Australians are wary of Celtic passion lifting unheralded Scotland.

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