Lewandowski said after that game he was confident of facing Real, but his struggles in training on the eve of the match suggest he faces a battle to be ready.
"He trained today only for 20 minutes, because he had a problem in his last game. He felt good, but we'll see how he feels tomorrow," said Ancelotti in a press conference on Tuesday.
"If he has no pain, he will play, if he has pain, he won't play."
Ancelotti refused to be pressed when asked what the chances are of Lewandowski facing a Real side led by superstar forward Cristiano Ronaldo.
"I can't give a percentage, he trained today and we have to see how he feels," said the Italian.
"He had a good feeling, but I can't say whether he will feel better or not (tomorrow)."
If Lewandowski drops out, his place is likely to go to Thomas Muller, who netted twice in the 6-0 drubbing of Augsburg a fortnight ago but has missed the last two games with an ankle injury.
"If he (Lewandowski) doesn't play, it won't affect our preparations and plans," said Ancelotti.
"We know how important Lewandowski is to us, but we also have other good players."
Winger Arjen Robben said the team was not nervous about the prospect of losing the star goal-scorer.
"The advantage we have is that we have a big team at our disposal, if he doesn't play -- which isn't sure yet -- we have other players who can step in which is why we are calm about it," insisted Robben.
