Shell-shocked Real Madrid coach Carlo Ancelotti described the 4-0 defeat at derby rivals Atletico Madrid on Saturday as his worst at the club and blamed himself for the capitulation.
A weakened Real defence, missing the injured Pepe and Sergio Ramos along with Marcelo through suspension, was torn apart by a rampant Atletico.
Stand-in centre backs Raphael Varane and Nacho Fernandez showed a lack of experience but they were also not helped by their teammates who failed to get back and provide support.
Increasingly erratic goalkeeper Iker Casillas was also at fault for the first goal as Real lost for only the fourth time in the league this season.
"I need to have a cool head to analyse what happened. I accept responsibility, it was the worst game that we have played since I have been here," Ancelotti, who took over in 2013, told a news conference.
"It is very simple, Atletico deserved to win because they were better than us in all areas. They fought more, they had more intensity and their play was better...everything.
"I am sorry. We are going to have to change our attitude and make sure it doesn't happen in another game. We made a lot of mistakes and we were always second to the ball."
Victory could have seen La Liga leaders Real open up a 10-point gap over their neighbours but instead the champions now lie four points behind on 50, level with second-placed Barcelona who visit Athletic Bilbao on Sunday.
"We played badly but we are still top of the table and I don't want to make too much of a fuss about this game. It was very bad but I have belief in the players," Ancelotti added.
There was little creativity in midfield where Toni Kroos looked weary while Cristiano Ronaldo, who was expected to be fresh after missing the last two games through suspension, was largely anonymous in attack.
Fellow striker Karim Benzema denied the loss to bogey side Atletico, who have got the better of Real on each occasion since their Champions League final defeat, was due to a lack of motivation.
"It was a disaster but it was not down to the fact that we weren't trying to win," he told reporters.
"It was a very bad game. It is always difficult to play against them but now we need to work hard to win the next game."