The Italian said as much after his side's controversial late win over Elche on Wednesday night, via football-espana.net:
I'm not happy. We need to play differently. We suffered a lot during the match, particularly during the closing minutes. We need to play better. We won’t win the derby if we play like we have done in this match, that’s why I think we need to improve, play with more intensity and personality.
Will it signify changes for the clash against Atleti at the Bernabeu? Probably not.
Ancelotti has shown a preference for starting his senior players, opting to leave his youngsters on the bench. That was never more obvious than the win over Elche, when the bench—not including Iker Casillas—had an average age of 20.83.
So while he's indicated his unhappiness at the way his Madrid side are playing, he's more likely to look for a reaction at the weekend rather than make a statement in terms of personnel.
Diego Lopez will presumably continue in goal, even if Casillas returned to the bench Wednesday night after missing the win over Getafe all together following the injury he sustained against Galatasaray.
Despite more options gradually becoming available, the same back four should start for the second consecutive match.
Fabio Coentrao returned against Elche and didn't look lacking in fitness too much, while Sergio Ramos, Pepe and Alvaro Arbeloa should maintain their places alongside him—Raphael Varane and Dani Carvajal will have to settle for a place among the substitutes.
Midfield might not be quite so straight forward.
Luka Modric and Sami Khedira started against Elche—with Xabi Alonso still sidelined—but Asier Illarramendi does present an alternative. Ancelotti will have to decide whether it will be him or Khedira who partner Modric against Los Rojiblancos.
"I think he will be ready for Saturday no problem. He was a bit sad after the Getafe game but changed quickly because he started to think about Saturday's game."
Angel Di Maria is the most likely option to drop out, with Bale lining up alongside Cristiano Ronaldo and Isco for the showdown between two of La Liga's unbeaten teams.
Karim Benzema finds himself under increased pressure to keep his slot as the club's first choice No. 9, but he still holds court with Ancelotti and Zinedine Zidane. However, another poor performance this weekend could open the door for Alvaro Morata or Jese Rodriguez.
Regardless of who starts the match, Madrid cannot afford to be as lacklustre as they have been in some recent league matches; Atleti, with renewed vigour after their Copa de Rey success at the Bernabeu, will almost certainly not be as forgiving.