Will Genia sustained a broken arm in Australia's second-Test defeat to Ireland on Saturday, with coach Michael Cheika critical of the "king hit" that caused the injury.
Genia is set to have surgery on Sunday following an off-the-ball incident involving Cian Healy that is likely to spell the end of his season, while Adam Coleman suffered a suspected fractured cheekbone.
The Wallabies scrum-half lasted only 26 minutes of the 26-21 defeat in Melbourne and left the field in clear discomfort.
"We ran the play down the front, Willy's the decoy around the back, and he got king hit from the side. It looked like a shoulder," Cheika said of Healy's challenge.
"And when you're not expecting it, that's what happens.
"Hoops [Michael Hooper] went down the front, Willy went around the back, I'm not sure who it was from the other team, [but they] came in and hit him and he jammed his arm and broke it."
Genia spent a month out with a knee injury earlier in the season.