McMahon had to cancel a holiday in Mexico just to be a part of the Wallabies' five-game tour of Europe and now sits on the cusp of the biggest moment of his career – a Test debut – after a superb performance against the Barbarians. McMahon was in everything for the Wallabies, starring in a 40-36 win at Twickenham to add another highlight to a year already boasting Australian junior captaincy duties, a rugby sevens bronze medal at the Glasgow Commonwealth Games and Australia's Super Rugby rookie of the year.

The 20-year-old now looms as the likely back-row replacement for Melbourne teammate Scott Higginbotham, who injured his hamstring in the first half and is in danger of missing the rest of the tour.

"To be told I was going to get an opportunity was just breathtaking," McMahon said. "I knew I had a good year and I was stoked with that, but I booked a trip [to Mexico] and when I got the call up … I'm still a bit speechless, I think. I'm still in a bit of a dream at the moment, just pinching myself every day. Just getting through and learning as I see it. Right now I'm just stoked to be where I am."

McMahon's family will consider making a dash to Cardiff if the Brisbane-born flanker is called into the Test team to play Wales. His father, Pat, is a security specialist working in West Africa at the mines in Gabon, travelling on stints of three months at a time, and his two brothers, Sam and Adam, also work in security in Australia.

The talented and powerful No.7 did not look out of place against the Barbarians, breaking through defenders with his first run of the game and constantly scrambling to save the Wallabies from conceding tries. He is also in charge of the team's mascot, Wally the Wallaby, as the youngest player on tour.

But just two weeks ago McMahon was so far out of Test contention that he was packing for a trip to Mexico with some of his Melbourne teammates to wind down after a hectic year.

"I got some credits for the flights, so that was good," McMahon said. "I'm just going to keep learning the structures and hopefully working towards getting a chance next week, that's what I want to do.

"[The tour] was definitely about the experience but, as every Australian wants to do, you want to get that first Test cap. I was hoping for an opportunity … if I get it, I'll do what I do best and that's just playing footy."

McMahon grew up playing rugby league in Brisbane before making the switch to rugby and rising through the ranks. Michael Cheika described McMahon as "outstanding" after playing 61 minutes in surprise extended game time.

"At the John Eales Medal he said he was doing his fitness work … he got on the field and took his opportunity with both hands," Cheika said. "With the injury to Scott Higginbotham, there's place in the back row open for the Test next week and he's put himself right up there."

Higginbotham will be given two days to recover from a hamstring injury before a decision is made on his fitness for the rest of the tour. The Wallabies suffered from a lack of depth of openside flankers at the World Cup three years ago, with David Pocock the only No.7 picked in the initial Wallabies squad for the 2011 tournament.

Australia now boast incumbent captain Michael Hooper, injured Pocock, Barbarians skipper Matt Hodgson, McMahon and Liam Gill.

"There's always a No.7 nipping at everyone's heels in Australian rugby," Hodgson said. "It's great, the depth in the back row is strong and it's good [for McMahon] to step up on that stage and test himself against a quality side. He did great for the 60 minutes he was out there. It's good to finally play football, that's what we enjoy doing and that's what we're out there for. We'll enjoy the win and then get back to work."