The Wallabies are facing the biggest challenge of their World Cup campaign with Hooper suspended for the crucial pool match against Wales while Israel Folau (ankle) and Rob Horne (shoulder) are set to miss the game.

Hooper was shocked when told he was cited, but pleaded guilty and was slapped with a one-week ban for his illegal cleanout on England fullback Mike Brown last weekend.

The Wallabies privately feared the incident could end Hooper's campaign with the penalty varying from two-10 weeks on the sideline.

But the silver lining is that Wallabies vice-captain will be free to play in the quarter final next week after pleading guilty and having his penalty downgraded.

The Wallabies have had minimal disruptions in the tournament so far, but injuries and suspension has thrown down big hurdles as they aim to finish atop the pool of death.

"It's always a shock to get cited … obviously I'm gutted I won't be able to be in the mix for selection. It really hurts," Hooper said.

"You always want to be in these sort of games – the big ones. It's really disappointing."

McMahon and McCalman are vying for Hooper's starting spot while Kurtley Beale and Drew Mitchell loom as the replacements for Folau and Horne.

Australia's depth at openside flanker is perhaps the strongest of any position in the team and Hooper's absence will be capably covered.

Hooper pleaded guilty after reviewing his collision with Brown in Australia's 33-13 win and independent judicial officer Alan Hudson took two hours to deem the offence on the low end of penalties.

Hudson took mitigating factors into account, including: good conduct during the hearing, good character and the absence of any off-field aggravating factors.

"Every match has been important through us throughout the year … we've built momentum through each game and this weekend is no different," Hooper said.

"Whoever puts on the jersey is going to have to do the job, fill the role and do it well.

"Certainly in this tournament style plays [there are hurdles], that's what I'm starting to learn. You have these unexpected things pop up."

Hooper has been one of the Wallabies' most consistent players over the past two years and will leave a huge hole in the starting side after forming an openside flanker tag-team combination with David Pocock.

Youngster McMahon and in-form McCalman will battle for a back-row spot alongside Pocock for the clash against Wales at Twickenham on Saturday night (Sunday morning AEDT).

McMahon and McCalman were equal best on ground performers in the Wallabies' 11-try rout against Uruguay two weeks ago.

"[McMahon] was here on his first Wallaby tour last year and we saw from the outset the guy was really up for it, really keen to get in and show what he's worth," Hooper said.

"He's shown that in games against the US and Uruguay, how much he is bringing to the team. Someone like Ben McCalman had a fantastic spring tour last year and again is impressing everyone when he's on the field recently."

The Wallabies are also bracing to be without Israel Folau (ankle) and Rob Horne (shoulder) as Michael Cheika's squad faces its biggest test.

Beating Wales will see the Wallabies finish at the top of the pool and potentially give them a smoother path through the World Cup play-offs.

The winner of the Wallabies-Wales clash will play either Scotland or Japan in the quarter finals and avoid New Zealand and South Africa until the final.

Hooper conceded he was surprised when told he had been cited almost two days after the 20-point win against England.

"With pleading guilty I know I was in the wrong. I didn't think it needed to go further than that [on the field], but I had a very fair trial and one week is going to be tough for me," Hooper said.

"You never want to miss a Test, but then again we've seen [other players] get hefty [suspensions] and you don't want to be involved in that.

"It's a tough part of the game, small margins for error and you can get it wrong, then you're in a situation I'm in or worse," Hooper said.

"It's a good example for [teammates] to keep working on that technique during training and not be in the situation I am now."