Mitchell Moses' fitness is Australia's main concern after the Kangaroos came from behind to beat New Zealand in their tournament opener on Thursday.
Coach Kevin Walters suggested the Parramatta halfback could be replaced for the rest of the tournament.
"We can bring Ethan in if Mitch is ruled out for the tournament, so we'll see how that goes in the next couple of days," Walters said.
Ethan Strange is among three shadow players, alongside Hamish Stewart and Tom Jenkins.
Walters confirmed Moses and Cameron Munster will miss next week's match against Fiji, while Sam Walker and Tom Dearden will form the halves pairing.
"He'll (Walker) play next week. We'll put him alongside Tom, who will play five-eighth," Walters said.
James Tedesco will return at fullback after missing the opener, having been overlooked before his niggle became known.
Walters said players who had competed in the Grand Final had earned a rest, adding: "I thought what was best for Australia was that they would have a spell, then they could enjoy getting the Australian jersey back on."
Lindsay Smith, the other squad member not named against New Zealand, is also expected to play. Bradman Best could feature on the wing after training there; Jack Bostock and Tom Trbojevic are the squad's other wing options.