But Wallaby coach John Connolly confirmed on Monday that Gregan is certain to start the match alongside his long-time team-mate and fly-half, Stephen Larkham, who will make his 100th Test appearance.
"It's a great occasion, you don't often get a guy who's [played] 130-plus [Tests] and another guy playing his 100th Test," Connolly said.
"The contribution they've made to Australian Rugby is phenomenal and they're virtually icons of Australian sport."
Edinburgh-bound Larkham will join Gregan and former Wallaby winger David Campese as only the third Australian to win 100 international caps.
Larkham's 100th Test will also mark the end of his career on home soil, 11 years after making his international debut against Wales in Sydney in 1996.
"It's a very proud milestone for myself but to have George (Gregan) alongside me is going to make it doubly special," Larkham said.
"I think sharing the last Test in Australia with George is also going to be very special and I'm very happy that he's going to be fit for the Test."