The Wallabies rookie looking to repay Dave Rennie's faith

After recovering from two serious knee injuries in his early 20s, Rob Leota is just thankful to be a part of the Wallabies squad.

So it was much to his surprise when he got the nod to start in the back row for Saturday's Rugby Championship clash with Argentina, after earning his first cap off the bench two weeks before against the Springboks.

"It was crazy, I got caught off guard," the imposing back rower said.

"I didn't really get any comms before it, it was a pretty unreal moment and to be able to tell my family as well.

"My face just popped up on the screen (in the team meeting) and I just froze for a bit...and it didn't really click until the boys started clapping."

Sporting a mullet, Leota comes across as a softly-spoken, gentle giant.

But on the field it's the opposite, which is why coach Dave Rennie has faith the 190cm, 110kg powerhouse from the Melbourne Rebels will be a force to be reckoned with against an equally-imposing Pumas pack.

"He's a big man, strong ball carrier. Has trained really well and has been very close to selection in the past," Rennie said.

"We're pretty keen to reward Rob - he's been with us full time - and that's a big part of the decision."

It's that backing that has Leota confident ahead of his run on debut for Australia and spurs his inner-belief that he can fulfil his coach's investment.

"When you've got a coach who believes in you, even though I got caught off guard, I know there's a reason why I'm there," he said.

"It gives me a lot of a lot of confidence that the coaches really believe in me, back me to do a job and to fulfil my role."