Robbie Deans has presumably hatched a plan but his Wallabies charges insist they have "no idea" what reshuffled backline the shrewd coach will field for Saturday's Rugby World Cup crunch match with Ireland.

Or more specifically, who will be charged with marking up against Irish superstar Brian O'Driscoll, one of the toughest assignments in international rugby.

As classy utility Adam Ashley-Cooper put it on Tuesday, strike winger Digby Ioane's broken thumb has "thrown a bit of a spanner in the works" and left Australia's contenders for centre and wing spots totally in the dark.

"Like always, it's like if your number is drawn out of a hat with Dingo (Deans)," Ashley-Cooper said.

"So I'll find out what number I wear, I guess tomorrow, but I'm hoping it's somewhere in the starting XV."

Ioane was due to undergo surgery on Tuesday night and his absence, particularly because of his role as playmaker Quade Cooper's cover as midfield defender, has triggered all sorts of conjecture over what Australia's best backline alignment might be.

Ashley-Cooper, James O'Connor and Drew Mitchell are all possibilities to start on the wing, while Ashley-Cooper is also being considered for outside centre, along with Anthony Faingaa, who started in Sunday's 32-6 win over Italy.

Ashley-Cooper and Faingaa are queuing up to directly oppose O'Driscoll for the first time.

"I'm sure whoever gets the 13 jersey this week will be very excited to face Brian," Ashley-Cooper said.

"Because that's what you want to do. If you want to be the best, you've got to compete against the best and Brian has been on top of his game for so long, he's regarded as one of the best centres in the world."

At 33, O'Driscoll has achieved just about every honour in the game - except win a World Cup.

He has won two European Cups with Leinster, captained Ireland to only their second Five/Six Nations Grand Slam in 2009 and three other Triple Crowns, earned a record 114 caps and, last March, broke the 78-year-old Five/Six Nations tryscoring record with his 25th five-pointer.

He was also named as Rugby World magazine's world player of the decade.

"He's a superstar of the game. He's done everything, he's been around for a long time and I guess that's the sign of a true great player of this game is longevity and consistency and he's been exactly that," Wallabies halfback Will Genia said on Tuesday.

Crash-tackling Faingaa was hoping he'd get the honour of marking his idol.

"It's going to be a fantastic opportunity if I get the chance to play against him," Faingaa said.

"He's been one of the standout players over the last 10 years and to get that opportunity for myself at a World Cup would just be great.

"Brian directs the play and the defensive play, he's someone that I really idolise and someone who has been great for rugby.

"Brian is just the kind of guy who slips out of tackles. I've been watching footage of him over the last five years and it seems that even when you think you've got him tackled, he gets out of it."

Deans will name his team on Thursday.

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