Heavy rain in the Russian capital over the past few weeks has seen the playing surface at the Khimki Arena soak up plenty of water ahead of the Group D encounter.
Despite UEFA officials confirming that the match will go ahead, Pellegrini is concerned that the fixture may be called off at the last minute.
"I've just seen it and it's unbelievable that the most important competition in the world is allowed to be played on this pitch," BBC Sport quotes Pellegrini as saying. "We must be praying tomorrow because if it rains it will be impossible to play.
"It is unbelievable that we can play on this pitch. I think the most important competition in the world deserves a better pitch."
City are currently in second place in Group D with three points from their opening two matches.