Atletico Madrid have announced that Sime Vrsaljko has partially torn a cruciate ligament in his left knee, though it appears he does not require surgery.
The right-back went down in the first minute of Sunday's 3-1 LaLiga win over Sevilla after attempting to make a tackle, and a subsequent MRI scan confirmed damage to his left knee.
Further tests have shown that the problem is not as serious as initially feared, although he is a major doubt for Atletico's next six games, including the Champions League quarter-final tie with Leicester City.
"Sime Vrsaljko underwent an MRI on Monday morning that revealed a partial tear of his posterior cruciate ligament and a grade I-II sprain in the internal collateral ligament of his left knee," an Atletico statement confirmed.
"The Croatia international defender will face physiotherapy and rehabilitation treatment to recover from his left knee injury."
Early reports in Spain suggest the former Sassuolo defender will be out for a month, ruling him out of the derby against Real Madrid and making him a major doubt for the second leg against Leicester on April 18.
He will also miss Croatia's World Cup qualifier against Ukraine and the friendly with Estonia on March 24 and 28 respectively.
Atletico went on to beat Sevilla thanks to goals from Diego Godin, Antoine Griezmann and Koke, narrowing the gap to third-placed Sevilla to two points thanks to their victory.