The two-time World Cup talent was injured in Friday's defeat of the Highlanders, Reds coach Les Kiss on Tuesday confirming he was set to go under the knife.
"Likely surgery ... likely be out for the season," Kiss said ahead of Saturday's clash with the top-of-the-table Blues.
"He's disappointed. It's a blow, without a doubt, a player of his calibre and the type of game he brings.Â
"Couple that with (the suspended) Fraser (McReight), Tate (McDermott), Seru (Uru, thumb) ... some good players unavailable."
It's Petaia's second season-ending shoulder injury. He also suffered a long-term foot injury and battled thigh, hamstring and concussion setbacks in a six-year professional career.
The 31-Test Wallaby is off contract beyond this season and has been linked to an NRL move to St George Illawarra, as well as rugby moves to Japan and Europe.
"Those are conversations with RA, but we'd like to ensure he stays around. I don't think it complicates anything in my head," Kiss said of retaining Petaia.
"He's a good rugby player ... hopefully he's all right by the end of the season and could do some more things then."
Meanwhile, Kiss said Josh Flook was a chance to play the Blues as he manages his own "acute" shoulder injury that saw him miss last week's 31-0 victory.
And influential flanker Fraser McReight has had his suspension for a high tackle reduced from three to two weeks after completing World Rugby's coaching mitigation course.
That means the Wallabies back-rower will be available for next week's clash with the Crusaders.
The Blues (7-1) await on Saturday though, fresh off 46-7 and 50-3 thumpings of the ACT Brumbies and Western Force respectively.
"They can go through the middle, through the edges," Kiss said.
"It was fairly compelling what they did to the Brumbies on the weekend.
"You look through it, there's not a lot of weaknesses."
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