The Ajax manager acknowledges that Los Blancos have more quality than his team, but he remains confident of his squad securing a surprise victory.
The 42-year-old Ajax boss, who had a four-year spell at Los Blancos' archrivals Barcelona during his playing career, thinks that despite the odds being stacked against his side, anything is possible if the players believe in themselves.
"There's a big chance Real Madrid will win, because they've got more quality and experience, but if we play our own game, you never know," the former Barcelona player told reporters at a press conference. "It is all about believing in yourself. Against Real Madrid last year, and Manchester United, we've shown that we are able to keep up.
"We must be very careful with the ball when we have possession. We did well last weekend against FC Twente, as they were waiting for us to lose the ball. But Real Madrid are so effective on the counter. That was very clear last year at the Bernabeu when they beat us 3-0."
When asked whether he thought the Madrid coach Jose Mourinho would be side-tracked by the prospect of El Clasico looming on Sunday, de Boer said, "The Champions League is really important for them. Last year they won the title in La Liga, and now they're aiming for this title. I don't think Real and Mourinho are thinking about El Clasico. The press does, but that does not mean that they do too."