The Blancos skipper believes Jens Keller's side can harm his side on the break and has told his teammates to produce a professional performance in both legs.
The Spanish giants easily qualified from a group containing Juventus and Galatasaray, while Jens Keller's side needed a 2-0 home win against Basel on matchday 6 to pip the Swiss outfit to top spot.
However, Casillas believes his teammates must be wary of the Gelsenkirchen side because of its prowess on the counter attack.
“Maybe they are not one of the favorites to win the Champions League, but we will have to play very professionally and respect the other team," he told the club's official site. “Schalke are very strong physically and play well on the break. We have the advantage of the second leg at home but we’ll go to Germany to win and try to go through to the next round.”
Madrid vice-captain Sergio Ramos, meanwhile, has echoed his skipper's calls for a professional performance and has spoken of his love for club football's top tournament.
"We're looking forward to the game because it's a competition that we really love," Ramos said. "Like all Real Madrid fans, we're hoping to go as far as possible in the competition and we'll take this game seriously and respect our opponents."