Real Madrid failed to detect the full extent of the injury suffered by Karim Benzema in last week's Champions League quarter-final against Atletico until he pulled out of Friday's training session.
Benzema limped off in the goalless draw at the Vicente Calderon last Tuesday after hurting his knee and was replaced by Isco in the second half.
The France forward underwent tests with Madrid the following day, but those examinations did not show any serious damage and the club doctor ruled out further checks.
The striker's injury was confirmed as a simple sprain, but Benzema felt discomfort in training again on Friday and was withdrawn from the session immediately.
Ruled out of the game against Malaga, coach Carlo Ancelotti assured his centre-forward would be ready to face Atleti on Wednesday but fresh tests showed the 27-year-old has ligament damage and Madrid finally confirmed the extent of his injury in a medical report on their website on Tuesday.
"Following tests carried out on Karim Benzema at the La Moraleja Sanitas Hospital by Real Madrid Medical Services, he has been found to have a sprain to the medial collateral ligament of his right knee," a statement read. "His situation will be monitored."
However, midfielder Luka Modric is out for six weeks with a similar injury and Benzema would have been unlikely to make it back in time for the second leg against Atleti even if the full extent of the problem had been identified earlier.
Nevertheless, at this stage of the season and with several other players sidelined, important time was lost due to an oversight by the medical team and it remains unclear how long the France forward will be out.
"Benzema got injured on Tuesday and on Friday it seemed as if he was alright," Ancelotti said on Tuesday when asked about the situation. "He tried to work on Saturday and Sunday, but he didn't improve.
"He has a sprain. Nobody was confused. He just has quite a serious injury and he hasn't been able to recover."
