Real Madrid captain Raul has suffered a complex knee injury that will require specialist treatment and is likely to be sidelined for at least two months although he could be out of action for much longer.
A scan carried out at the Ruber International hospital on Monday showed the 28-year-old striker had suffered a partial tear of the cruciate ligament in his left knee and also damaged the cartilage.
Raul sustained the injury when shooting during his side's 3-0 defeat by Barcelona on Saturday and had to leave the pitch 13 minutes into the second half.
"If it is only the cartilage that is affected then the recovery time will be between a month and a half and two months," said club doctor Juan Carlos Hernandez.
"But if the cruciate ligament tear is serious then he could be out for much longer.
"We will have to consult the specialists because the images in the scan were not clear, so we need to gather as much information as possible."
Spanish media suggested that the player's participation in next year's World Cup might be in danger if the injury is as serious as feared, but Dr. Hernandez was more upbeat.
"He's been playing at the top level for many years and he's never been injured and that will work in his favour," he said.
Raul's strike partner Ronaldo may also miss Wednesday's Champions League game at home to Group F leaders Olympique Lyon after suffering a reaction to an ankle injury following his return to action in the Barcelona match.
"We've been really unlucky with injuries," said Raul's team mate, goalkeeper Iker Casillas.
"It isn't normal what is happening. We all hope that Raul will recover as soon as possible because he is very important to us. His absence comes at a difficult time for us and we need him."