Julio Sergio pulled up with a potentially serious thigh strain during the first half.
His replacement, Alexander Doni, had a hand in some of the goals scored by Panathinaikos.
"We must wait for tomorrow to see the entity of Julio Sergio's injury. We know how important he is for us and now Doni will have the opportunity to prove himself," said Ranieri.
"It was difficult this evening and he seems to struggle in the night games with the ball travelling that fast. I'll soon have a chat with him."