De Gea told The Sun: "City possess dangerous forwards and any of them are capable of scoring against us.
"I know Aguero well and in this type of game he is always at his best. Yes, the City attack has its stars but I wouldn't swap Wayne for any of them. He is more than a Manchester United footballer - he is the key man in our squad.
"Wayne is the same player in training sessions as he is in the matches. He always gives his maximum.
"He really is a worldwide star and I notice just how focused he is on this derby. This derby is a final for us. If we beat City, United will be Premier League champions again. It's as simple as that - we are 90 minutes from winning the title.
"I won the Europa League final with Atletico Madrid but this is now the most important day in my career. I never believed this derby would be so decisive.
"I have lived through derbies in Madrid and this one is more important, especially with a title resting on it."