The Reds may be without defender Martin Skrtel for the Round of 32 second leg at Anfield after he sustained a knock on his knee.
"He is being assessed but I think he'll struggle," said Rodgers.
"He got a little knock on his knee a few days ago and tried to train on it but made it worse. So it's highly unlikely that he will play."
Liverpool lost 2-0 at Zenit last week so will need to overturn that deficit in order to move through to the next phase of Europe's second tier club competition.