Atletico Madrid coach Diego Simeone says the injury to goalkeeper Jan Oblak was the worst thing about Monday's damaging 3-0 defeat to Villarreal.
The home side took a 2-0 lead into the break thanks to goals from Manu Trigueros and Jonathan dos Santos at El Madrigal, with Tiago and Oblak culpable for those respective strikes.
But Oblak was forced off with a suspected shoulder injury sustained while conceding Dos Santos' goal, with Miguel Angel Moya taking his place.
Roberto Soriano ultimately wrapped things up in stoppage time to give Villarreal a comprehensive win and leave Atletico 12 points adrift of pacesetters Real Madrid, but after the match Simeone's main focus was on his Oblak.
"The worst thing is [the injury to] Oblak," he told reporters. "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."
The defeat was Atleti's third in five LaLiga outings and may prove a decisive blow in their title ambitions, despite the campaign having not yet reached the mid-season break.
"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 added.
"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."
Captain Gabi also had his say after the match's conclusion and was left ruing Atletico's individual mistakes, before warning that they need to make significant improvements if they are to finish anywhere near the top of the table.
"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."