Santos are in the tournament by virtue of winning the Copa Libertadores, and could meet the reigning UEFA Champions Leaue champions in December should both sides win their semi-final fixtures in Japan.
Speaking to Sport, Leo said: "Barcelona are the best team in the world and are the clear favourites".
"They have so much quality and their passing is especially impressive. Their players are great. However, we should not forget that they are men, just like us, and not gods.
"On the pitch it will be 11 against 11 and that's where the game will be decided. We have to believe in ourselves and see how far we can go."
The draw for the Club World Cup is scheduled to take place on Nov 17.