Mensah hurt in Ghana training
Mensah went down during a practice game 24 hours ahead of their Group D meeting with Australia.
Ghana's Serbian coach Milovan Rajevac told reporters both Mensah and fellow-defender Isaac Vorsah, who is also injured, were doubtful for the game.
Rajevac, speaking through an interpreter, said: "We have other players to replace them."
He was unable to explain the injuries but Ghanaian reporters said Mensah may have suffered a repeat of the back-related injuries that he has suffered for two years.
Vorsah's injury was not explained.
Rajevac stressed the importance of the result and said he had warned his players against complacency following their 1-0 victory over Serbia.
"For us the result is all-important and we must be motivated by our opening win because Serbia won today (against Germany) and they are back in it now."
Striker Asamoah Gyan, who scored from the spot against Serbia, said: "In 2006 we did quite well, we went to the next stage but this time Ghanaians are expecting more from us.
"That is why we are also going like wounded lions. Like a wounded lion, you know? So I'm hoping we just qualify for the next stage and then we see what happens from there."