Atletico Madrid have announced that defender Jose Maria Gimenez is suffering from a muscle infection.
The Uruguay international has been forced to sit out the last two training sessions and is undergoing a course of antibiotic treatment.
The 22-year-old is therefore a major doubt for Sunday's La Liga clash with Celta Vigo at the Vicente Calderon.
"Jose Maria Gimenez could not train with his team-mates in the last two sessions due to 'an infection in the subcutaneous cellular tissue of the right leg, which does not affect joints nor other structures'," Atletico said in a statement.
"The Uruguayan defender is undergoing a shock treatment with antibiotic to attack the infection."
The news is a blow to coach Diego Simeone given Atletico's problems at the heart of defence.
Diego Godin was diagnosed with a thigh muscle injury on Thursday that will reportedly see him sidelined for up to two weeks, making the 30-year-old a doubt for the Champions League last-16 first leg against Bayer Leverkusen on February 21.
With Lucas Hernandez embroiled in legal proceedings and Gimenez also now out of action, Stefan Savic is likely to be Atleti's only recognised centre-back for Sunday's meeting with Celta.
Simeone's side sit fourth in the table, seven points behind leaders Real Madrid, who have two games in hand.