Zinedine Zidane says his team did not deserve to be beaten by a stoppage-time goal from Espanyol, as pressure mounts on the Real Madrid boss.
A much-changed Los Blancos, with Cristiano Ronaldo left out of the squad by Zidane, looked to have done enough to labour to a share of the points.
However, with more than two minutes of added time played, Espanyol broke clear and Gerard Moreno crashed the ball past Keylor Navas to give the Barcelona club a first win over Real Madrid since 2007.
Defeat leaves Real Madrid in third place, 14 points behind leaders Barcelona, who have a game in hand, and two ahead of Valencia having played one match more.
Espanyol's victory ended the visitors' seven-match unbeaten run, which had seen them pick up six victories and score 30 goals.
"We made a bright start and had some chances to score, but our second-half display was worse than the opening 45 minutes. We didn't manage to play at all or offer any sort of threat in the second half, although we didn't deserve to lose the game," Zidane said at a press conference, quoted on the club's official website.
"Espanyol created few chances. It's a defeat that hurts and it's a poor result. I feel sorry for the players."
Zidane, though, did accept his much-changed team, which had captain Sergio Ramos back following suspension, "were lacking a little bit in all departments".
The French coach continued: "At times, there's no understanding football. We dropped further back for some spells in the second half, but you've got to accept these tough times."
"At no point in the second half did we do well. After five good results, today we didn't manage to extend the run."
Zidane also refused to be drawn on what impact Neymar's foot injury would have on the second leg of their Champions League tie against Paris Saint-Germain.