Fainga'a was ruled out on the eve of the San Juan Test on Sunday morning (AEST) after concussion symptoms emerged from a Thursday training session.

It's a massive blow for Fainga'a who turned in his best Test performance in last week's 41-26 win in Mendoza.

The Wallabies have already lost Dave Porecki (concussion), Michael Hooper (mental health), Quade Cooper (Achilles), Hunter Paisami (concussion) and Allan Ala'alatoa (family bereavement) from the tour.

Lachlan Lonergan will make his first Test start while 20-year-old hooker Billy Pollard, who was a late call-up to Argentina following the loss of Porecki, is in line to make his international debut.

Pollard has only played eight Super Rugby games but gained some valuable experience with the Australia A team in the Pacific Cup last month.

It leaves Australia with two debutants among the reserves in Pollard and Pone Fa'amausili while fellow front-rower Matt Gibbon only played a few minutes of Test rugby from the bench in the first Test.

The late change to the front row will bolster hopes of a home-town victory with the Pumas priding themselves on their set-piece.

But stand-in skipper James Slipper said he had plenty of confidence in both Lonergan and Pollard.

"Lonners (Lonergan) has been playing really well this year and it is another good opportunity for him," Slipper said at their final training session.

"It's his first start for the Wallabies and one thing you know about 'Noss' is that he looks very calm, but when he's out there he goes full throttle so I am really excited to see how he goes."

He joked with Pollard's rushed flight to Argentina he didn't have his own boots.

"Billy obviously came over late and he has been using my scrum boots all week so I am not even sure if he has his own boots.

"He is a talented young guy who is really excited and you can tell how pumped he is so what you get from youth is that enthusiasm and I reckon both of them will bring heaps."

With former Brumbies coach Dan McKellar now steering the Wallabies forwards, Slipper said the transition from Super Rugby to the Test stage would help the two players feel comfortable.

"For the players coming in, it's not incredibly new to them," Slipper said.

"There is not much detail uptake to be done, so they're pretty comfortable with everything they need to know and having Dan here is a big help for them, having the same coach.

"They're pretty comfortable. They don't seem nervous at all but let's wait until tomorrow I guess."

WALLABIES: Tom Wright, Jordan Petaia, Len Ikitau, Lalakai Foketi, Marika Koroibete, James O'Connor, Nic White, Rob Valetini, Fraser McReight, Jed Holloway, Darcy Swain, Rory Arnold, Taniela Tupou, Lachlan Lonergan, James Slipper (capt). Reserves: Billy Pollard, Matt Gibbon, Pone Fa'amausili, Nick Frost, Pete Samu, Tate McDermott, Irae Simone, Reece Hodge.

Australian Associated Press