Diego Costa suffered a foot injury after scoring his first LaLiga goal of the season against Barcelona, says Diego Simeone.
That had Atletico on track to leapfrog Barcelona into top spot in the LaLiga table, but Barca hit back to deny Simeone his first LaLiga win in this fixture during his seven years as Atletico coach.
Costa had already been withdrawn, though, and Simeone confirmed the Brazil-born Spain striker had asked to be substituted due to injury.
"He retired with a little discomfort in his foot and asked for the change," Simeone said to Movistar.
Costa could therefore be a doubt for Atletico's next match, at home to Monaco in the Champions League on Wednesday.
But Simeone will want Costa to shake off the injury, with his first goal in 10 LaLiga appearances against Barcelona potentially a turning point in his season.
"We hope so," Simeone said of Costa's previously poor form being boosted by his header.
"He's an animal - and every time he's on the field, the team has a chance to win."
Atletico stifled Barcelona for much of the top-of-the-table contest, with only two shots on target recorded by either side.
"The style was very similar because we both played something similar," Simeone said.
"I liked everything, we had a commitment, we were close to winning."