The 23-year-old openside, far and away the team's best player last season, has suffered a stress fracture in his lower back and will miss Saturday's season-opener against the New South Wales Waratahs and at least three other matches.
The Reds, 13th last year with four wins and 12 defeats, can ill-afford the loss of a player of Gill's quality after the departure to Europe of Will Genia, Quade Cooper and James Horwill -- the mainstays of their 2011 title triumph.
Coach Richard Graham was already without captain and prop James Slipper, who has had shoulder surgery, and test lock Kane Douglas, who sustained a knee injury at the World Cup.
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Gill, capped 15 times by the Wallabies, will be moving to France at the end of the season, the Reds said two weeks ago.
There was potentially more bad news for Graham when it was announced on Thursday that Rod McCall would be stepping down as chairman of the Queensland Rugby Union with immediate effect.
World Cup-winning Wallaby McCall, who was an enthusiastic supporter of Graham despite the coach's poor record in his two seasons in charge, departs for personal reasons and will be replaced by Damien Frawley.
(Reporting by Nick Mulvenney; editing by Ian Ransom)
