Cristiano Ronaldo sprained his left shoulder during Real Madrid's 2-2 clash with Barcelona on Sunday, the club said, placing him in doubt for the Portugal World Cup squad this week.
"Cristiano Ronaldo suffered a sprained left shoulder. He will undergo more tests with doctors over the next few hours," Real Madrid club said in an online statement.
Ronaldo, who scored both of his club's goals at Barcelona's Camp Nou stadium, landed badly on his shoulder after trying to score with an overhead kick in the 64th minute.
He could be seen grimacing and gripping his shoulder as he sat on the pitch after the injury.
"Cristiano could be off for two weeks and miss his call-up for Portugal," said Madrid-based sports daily AS.
Ronaldo was due to join with the Portuguese side on Monday to prepare for Brazil 2014 World Cup qualifying matches against Russia this Friday and against Northern Ireland on October 16.
According to AS, Real Madrid hopes that the Portuguese federation will accept a medical statement on his condition so that he can be excused Monday's call-up.