Diego Simeone indicated the injury Joao Felix suffered in Atletico Madrid's LaLiga draw with Valencia on Saturday "could be serious".
Joao Felix, who joined Atletico in a €126million transfer in the close season, hobbled off with 10 minutes left at the Wanda Metropolitano after twisting his ankle in a tussle for the ball with Dani Parejo.
The injury left the host, who had already used all three substitutions, with 10 men and Valencia quickly added insult to injury as Parejo bent home a stunning free-kick to equalise and claim a 1-1 draw.
Atletico confirmed Joao Felix will have tests to confirm the severity of the injury and speaking in his post-match media conference, Simeone said: "It did not seem much at first, but the doctors say it could be serious."
Diego Costa opened the scoring for Atletico with a 36th-minute penalty, but this was another game in which he and strike partner Alvaro Morata struggled to take their chances.
The stalemate left Atletico fourth in LaLiga having failed to win five of its past six matches. Simeone's men have scored just three goals in that run and have not been victorious at home in the top flight since a 3-2 defeat of Eibar on September 1, this latest setback giving leader Real Madrid the chance to go five points ahead of it.
"Costa and Morata know that the only path is to keep working, to insist. Forwards know that when they score goals they are fantastic, and when they don't people point fingers," added Simeone.
"We are a new team, and we don't yet have balance to manage moments in games, above all when we need to suffer and position ourselves well and keep calm. But we are convinced we have players to do this, there is work to do, and I like that.
"Nobody thought this would be an easy season, I knew we would have a lot of work to do, to get the team balanced.
"In certain moments some people look to point fingers, others to work hard. Those who want to work hard will get on well with me. Those who point fingers won't.
"We are in a very good moment to work hard, and look to see who wants to work, and who does not like to work."