Wallabies winger Adam Ashley-Cooper is almost certain to be ruled out of Saturday's pivotal World Cup clash against Wales.

Ashley-Cooper has rated himself a 50-50 prospect of being fit for the pool B match in Cardiff after hyper-extending his big toe on his right foot in Australia's 91-3 thumping of Japan.

But the powerful 23-year-old's inability to train and the strong form of Drew Mitchell is set to see the Wallabies selectors err on the side of caution when they announce their team on Thursday.

While he is keen to play in Cardiff, Ashley-Cooper realises there's no point rushing his return in the second week of the seven-week tournament.

"It's a massive game and out of all the games this is probably the one I wanted to play, against Wales," he said.

"But there are bigger fish to fry and if the toe isn't 100 per cent I'm not going to risk that. It's not worth it."

Ashley-Cooper scored the fifth of Australia's 13 tries in their opening-match rout of Japan in Lyon on Saturday but the second-half injury robbed him of a potential hat-trick.

"As soon as I came off Drewy's (Mitchell) first two touches were tries so he did very well but I don't look at it that way," he said.

"I was happy with the time I got out there and the way I performed."

Mitchell's likely ascension to the Wallabies starting XV is expected to be one of two changes from the side which hammered the Cherry Blossoms.

Prop Guy Shepherdson is poised to replace Al Baxter at tight-head if he proves a sufficient improvement made on knee and quad niggles.

The winner of the Cardiff clash will virtually nail down top spot in pool B and draw the pool A runner-up, likely to be the loser of Friday's blockbuster between South Africa and England, in the quarter-finals.

Wales have encountered Australia three times in World Cups and enjoyed one win - 22-21 in the third place play-off in Rotorua in 1987.

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