Genia underwent surgery on Sunday after suffering the injury during the second Test against Ireland in Melbourne on Saturday, a match the tourists won 26-21 to level the three-match series.
His absence is not only a blow for the Wallabies ahead of a decider in Sydney but also the Melbourne Rebels, with the 30-year-old now likely to sit out the remainder of the Super Rugby season.
Gordon has flown back from Japan – where he had played in an exhibition match for the Waratahs – to join up with the squad.
The 24-year old is yet to earn his first Test cap, after his initial squad call-up a year ago during the 2017 June Series.
Head coach Michael Cheika is expected to promote Nick Phipps to the number nine shirt in Genia’s absence.
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