Real Madrid forward Cristiano Ronaldo damaged his left shoulder in Sunday's 2-2 La Liga draw at Barcelona, the club said in a statement on their website.
Ronaldo, who netted both of Real's goals at the Nou Camp, landed heavily on the shoulder in the second half.
"The player is waiting for medical staff to conduct more tests in coming hours," Real said.
Ronaldo is due to meet up with the Portugal squad for their World Cup 2014 Group F qualifiers away to Russia on Friday and at home to Northern Ireland four days later.
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