Diego Simeone cited the injury to goalkeeper Jan Oblak as the worst aspect of Atletico Madrid's 3-0 defeat to Villarreal on Monday night.
A dominant Yellow Submarine side rode goals from Manu Trigueros, Jonathan dos Santos and Roberto Soriano to victory at El Madrigal, with the Slovenian being forced off with a a shoulder injury in the aftermath of Dos Santos' strike.
Villarreal jumped Atleti in the standings with the result, but Simeone's primary concern afterward was the health of the 23-year-old keeper.
"The worst thing is [the injury to] Oblak," Simeone said. "Hopefully it's not so serious, but his injury upset us.
"The second half was not easy because we were 2-0 down and the injury to Oblak, but the team tried everything that they could to reduce the score."
Atleti have now lost three over their last five matches in La Liga play. Though Simeone wouldn't credit Monday's defeat with a lack of effort.
"I go with the feeling that the team fought, worked and looked for the best chance to win, but the results are not encouraging us," Simeone said.
"We have to work because it is the only way to improve. We accept the criticism, because when a team loses games you need to improve."
Gabi, who acted as captain for the match, blamed individual miscues for the lopsided scoreline on the day.
"I don't think that Villarreal were better than us, we dominated the second half," he told beIN Sports. "But In the end when you make mistakes you pay for it.
"We gave our everything and we were always in it, but their goals came through individual mistakes.
"Obviously we have to improve. And to be up near the top [of the table] we will have to improve a lot."