The Ballon d'Or winner failed to score in Barcelona, but has notched 31 goals in 27 games for the Spanish outfit this season.
"He's a born winner and always wants more and to score more goals," Casillas told the club's official website.
"Even though he didn't score Tuesday, the whole team knows that he is important for us and will work hard to help the team in the second leg. I'm sure that he'll score. He is a cornerstone of this Real Madrid team."
Ronaldo has scored 232 goals in 226 games for Madrid since arriving in the summer of 2009.