Casillas is hoping Ronaldo can repeat his performance in the Copa when the Spanish side visit Old Trafford for their last-16 second leg next week.
"Cristiano is vital for us. Right now he's in sensational form and gives us his maximum," he said. "We have a 1-1 draw from the first leg against United and we have to score and keep a clean sheet.
"It will be difficult as they also know how to play and their ground is tough to play at. We have to transfer what we did here at Camp Nou to Old Trafford."