Ex-United star Cristiano Ronaldo scored a hat-trick in that game, as De Gea is acutely aware.
"Everyone knows how well he can play and it is very hard to save his shots," said De Gea. "No-one can hit shots like him. But we will do everything in our power to contain him."
When asked about his own form in this, his second season in England, the Reds' number one said: "Of course, when you arrive at a new club it is normal there will be some low points. It is important to improve and I am now much better than when I first arrived at United."
Speaking at the same press conference, Sir Alex Ferguson praised De Gea's progress. "David has shown his character because he has taken a lot of criticism, sometimes unfair criticism.
"But what we at United are good at is developing people, bit by bit. It's like a young child learning to walk. He moves forward, he wobbles. He gets up, wobbles, gets up again and now he's walking."
David De Gea - Manchester United
Born: 7 November 1990, Madrid, Spain
Joined United: 1 July 2011
Debut: 7 Aug 2011 v Man City
Former clubs: Atletico Madrid
International: Spain U-23s (Olympics)
Iker Casillas - Real Madrid
Born: 20 May 1981, Mostoles, Spain
Joined Real: 1990 as a youth player
Debut: 12 Dec 1999 v Athletic Bilbao
Former clubs: None
International: Spain (143 appearances)