But the 21-year-old is unfazed by the butterflies in his stomach or the star names he will crash into when the Wallabies play Wales on Saturday night (Sunday morning AEDT).

McMahon has vowed to throw himself into every contest with the same ferocity that has earned him a hard-as-nails reputation after just five Tests.

Wallabies coach Michael Cheika has called on his young gun to fill the "big shoes" left by suspended vice-captain Michael Hooper.

It is a moment even McMahon admits he thought was an impossible dream 12 months ago when he was a bolter in the Wallabies' spring tour squad.

He will wear the Wallabies' No.7 jersey and team up with David Pocock to take on Welsh captain Sam Warburton and breakdown expert Justin Tipuric.

And to get him up to speed, Pocock and Hooper have given the Melbourne Rebels hardman some expert guidance to ensure he's ready for the breakdown battle.

"For the Rebels I basically played [blindside flanker] and I've come to No.7 for the Wallabies, it's been a bit of a learning curve," McMahon said.

"I kind of knew how to play it because I played a little bit through school.

"To play alongside a world-class player like 'Poey' is going be an unbelievable experience. I haven't had much of a chance to do that so playing alongside him and feeding off him is hopefully only going to improve my game."

McMahon has a simple approach to stepping up for Hooper – "I'll just have a dig".

It is that mentality that has impressed Cheika and earned him a starting XV spot ahead of Ben McCalman.

McMahon is one of the most feared players at Wallabies training, showing no regard for self-preservation and unafraid to mix it with his older teammates.

He wants to transfer that into the match against Wales, promising to take on the physical and defensively aggressive role that Hooper has played.

Pocock's breakdown prowess will free McMahon up to do what he does best – be a physical menace.

"I've probably come out of a game feeling a few bumps and bruises that were probably from myself doing harm more than other players," McMahon said.

"But it's always good to bring that physicality to the game and if that's helping the team, that's good.

"To be honest, I'm just going out and doing what I do best and that's playing footy and have a dig."

Wales will attempt to combat the Wallabies' double flanker act by sending Warburton and Tipuric into the breakdown contest.

It's a strategy to negate the influence of Pocock and keep a lid on McMahon's power.

McMahon is the clear outsider in a group of word-renowned back-rowers. But you only have to ask McMahon's teammates to get an indication of how bright his future is.

"I don't think [holding back] is in Seany's repertoire," said lock Dean Mumm grinning.

"He's gung-ho, isn't he. And he's good … he deserves his opportunity because of how well he has played in games. I feel he will grasp it with both hands."

McMahon's Test debut was a shock appearance against Wales in Cardiff last year.

He said memory of that game was a blur – both good and bad things – but he's confident he won't be overawed when he steps on to the big stage.

"I'm excited and I've got the butterflies a bit, but they'll go away coming into the game," McMahon said.

"It's more just knowing I'm ready for the game and that build-up because you don't want to fear failure. Once you get that first touch, the nerves go.

"It's going to be a big battle [at the breakdown], we're going to be brutal at the ruck and bring that physicality at the ruck … we're ready for the challenge.

"I definitely didn't think I'd be here in 12 months, but being here is exciting and getting an opportunity to play another game is exciting and I wouldn't want to be anywhere else."