Real Madrid head coach Carlo Ancelotti was critical of his team's performance as they struggled to defeat Elche in La Liga on Wednesday.
A controversial stoppage-time penalty won by Pepe and dispatched by Cristiano Ronaldo saw Real Madrid scrape a 2-1 win over their plucky minnow opponents.
Ancelotti warned his side that if they played at the same level on Saturday, they will have no chance of beating cross-town rivals Atletico Madrid.
"Playing the way we did today, we are not going to win the derby," Ancelotti said.
"We need to play better, with more intensity and more personality.
"The second half we didn't have control of the game and at the end, Ronaldo scored but I am not happy with the way we played.
"The only important thing is that we won but is not enough, we need to improve."
Ancelotti was reluctant to discuss the referee's decision to award the match-winning penalty but said Pepe confirmed the spot-kick was warranted.
"It is difficult to talk about (the penalty) when you are on the bench, because you can't see properly," Ancelotti said.
"I talked to Pepe and he told me that it was a clear penalty."