Real Madrid vice-captain Marcelo said his team needed to learn from Wednesday's 2-1 La Liga defeat at Valencia.
Early goals by new signings Simone Zaza and Fabian Orellana put the home side 2-0 up in nine minutes at the Mestalla.
Cristiano Ronaldo pulled one back just before half-time, but it was not enough as Zinedine Zidane's side missed the chance to move further clear of Barcelona and Sevilla in the table.
Madrid remain a point ahead of Barca at the top, with Sevilla two points further back in third, but face a tricky trip to Villarreal on Sunday and have only won one of their last six away games.
Their defence was all over the place in the early stages, with centre-back Raphael Varane and goalkeeper Keylor Navas at fault for the goals.
And speaking on Movistar, Marcelo said his team had paid the price.
"We started the game very badly," the left-back said. "We gave away two stupid goals.
"But we must learn from our mistakes. We are still top of the table, but we must change -- we cannot start games like this."
Marcelo added that it was difficult to play at Valencia, where Madrid have taken just one point from their last three visits, adding: "Valencia have a great team, and the crowd pressure you a lot.
"We must learn from this mistake so it does not happen again. It is a defeat, but we must keep going.
"We did everything possible to try and equalise, but we must change some things."
After Madrid's last La Liga defeat, at Sevilla in January, Marcelo also spoke about the importance of learning from errors.