The La Liga champions travel to the Premier League leaders in the second leg of their last-16 clash having endured a 1-1 draw in the first leg at the Santiago Bernabeu.
Madrid suffered the same first-leg result in the Copa del Rey before going on to beat Barcelona 3-1 away in the return fixture, and Casillas is hoping for a repeat of that performance against United.
"Cristiano is vital for us. Right now he's in sensational form and gives us his maximum," Casillas said. "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. It is also 1-1 and we have to score and keep a clean sheet. That's the aim that we're going with."
Madrid travel to United on Tuesday.