Sime Vrsaljko faces a substantial spell on the sidelines after suffering a serious knee injury during Atletico Madrid's 3-1 LaLiga victory at home to Sevilla on Sunday.
The right-back went down in the first minute of the game after attempting to make a tackle and Atletico confirmed an MRI scan has shown he sustained an anterior cruciate ligament injury to his left knee.
Vrsaljko was replaced by Juanfran and Atletico went on to win the match thanks to goals from Diego Godin, Antoine Griezmann and Koke, narrowing the gap to third-placed Sevilla to two points thanks to their victory.
More tests will be required before a timescale is possible for Vrsaljko's recovery, but his injury will be a blow to Diego Simeone's men, who are due to face Leicester City in the quarter-finals of the Champions League.
Croatia will also miss Vrsaljko, who has become their first-choice right-back following the retirement of Darijo Srna, with Ante Cacic's men due to face Ukraine in a World Cup qualifier on Friday.