The club said in a statement on Wednesday it was hopeful Mortlock would still play after scans on his right leg showed ''minimal damage''.
If he fails to pass a fitness test on Friday, it will not be the first time the 35-year-old is forced to watch from the sidelines in what should be moments for him to savour.
He missed the Brumbies' Super 12 wins in 2001 and 2004 because of injury.
Mortlock, who also lost the Wallabies captaincy in 2009 after damaging his knee in a Tri-Nations clash against South Africa, last week announced he would retire after realising that his body would not take him through another season.
Mortlock will retire after the Rebels' final games against the Lions and Stormers.