The Spain international hopes the club's talisman will be fit in time to face Barcelona in the Copa del Rey final and is looking forward to facing Bayern in the Champions League
The former Manchester United ace, 29, suffered a knee injury that kept him out of Tuesday’s 2-0 defeat to Borussia Dortmund in the Champions League and will be absent for this weekend's clash against Almeria - and possibly even miss the Copa del Rey final against Barcelona on Wednesday.
The Spain shot-stopper says he hopes his team-mate makes a quick recovery.
"He's a big miss. We can't have him tomorrow and possibly not for the cup final either," Casillas told the club's official website.
"With the goals he gives us and his presence, Cristiano imposes himself on the opponent and is important for us. He will know if he’s going to be fit for the final.
"If he's better he will be the first to know. [On Saturday] he won't be there but hopefully we can win the game as we have a league in which we're off the pace, but we want to win to go back up to first."
Casillas then turned his attention to the Champions League semi-final, in which los Blancos have been drawn against reigning European champions Bayern Munich.
"Now is the most important part of the season and we have a big enough squad to go for all three titles. We go game by game.
"Against Bayern it will be an attractive game because they're a tough rival and the current Champions League holders. We have to try and beat the reigning champions.
"They have great players but we're going to give our all to get to the final in Lisbon. We had a bit of a scare in Dortmund, but we will play an exciting game and hopefully it goes our way."