Jamie Roberts' Test hopes will rest on the results of a scan after he was the major casualty in the Lions' record tour victory over the Waratahs.
The powerful centre failed to complete the game when he limped off 10 minutes from the end of the 47-17 success over the Super Rugby outfit.
Roberts, who recently qualified as a doctor from Cardiff University, looked crestfallen as he was treated on the sideline by team medics just a week out from the first Test against Australia.
Head coach Warren Gatland said a scan on Sunday will decide centre Roberts' fate.
"He's going to get a scan tomorrow. The medics thought it might have been a hamstring, but he said he has never pulled a hamstring before," Gatland said.
"He's not kind of sure, so he's got his fingers crossed that it may not be that. So we just have to see what happens tomorrow."
Roberts, capped 53 times for Wales, is rated as the most damaging running midfielder in the northern hemisphere and was set to combine with Brian O'Driscoll in Brisbane.
He formed a formidable centre partnership with O'Driscoll on the Lions 2009 tour of South Africa and was named the Lions' player of the series after top efforts in the first two Tests.
His injury may open the door for Welsh compatriot Jonathan Davies, who can play at 12.