Inglis won two World Cups with Australia and was part of the Queensland team that dominated State of Origin for a decade before ending his career at the Rabbitohs, who he helped win a first National Rugby League (NRL) title in 43 years in 2014.
The 32-year-old featured in two matches at the start of this season but 12 years playing one of the most physically demanding of sports had taken its toll on his body and he hung up his boots in April.
The Rabbitohs retained Inglis, one of the best Aboriginal Australian players to have graced the game, to work on the club's community projects and he would have been expected to have a big off-field role in the NRL Indigenous round this weekend.
"Rabbitohs and Souths Cares ambassador Greg Inglis has entered a facility to undergo treatment to assist with and support his mental health," the club, co-owned by Hollywood actor Russell Crowe, said in a statement on Friday.
"On behalf of Greg and his family, we ask the media and the public to respect their privacy.
"No further comment will be made by the Rabbitohs, Greg, his management or his family at this time."
The news will only intensify the ongoing debate about the responsibility of sports to help their players ease into retirement, a debate revived in Australia by the suicide of former rugby union international Dan Vickerman in 2017.
"The best thing I can say about Greg is that he's getting the support he needs," Rabbitohs coach Wayne Bennett told reporters on Friday.
Inglis was handed an 18-month good behaviour bond by a New South Wales court in January for drink driving in October 2018, a charge that cost him the captaincy of the Australia team he had only just been awarded and a two-test ban.
"I know where I stand in the game as a role model to the community," he said after apologising for his actions.
"I know I did the wrong thing and, while I am disappointed at missing the tour, I accept the penalty."
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