Cristiano Ronaldo a doubt for Portugal's World Cup qualifiers

Real Madrid star Cristiano Ronaldo could be ruled out for Portugal's 2014 World Cup qualifiers due to an injured shoulder.

The 27-year-old, who scored twice in Madrid's 2-2 draw with Barcelona at the Nou Camp last night, picked up the injury after falling awkwardly from an attempted overhead kick during the fierce encounter.

Madrid confirmed yesterday that the forward was due to undergo further tests but it is understood that Ronaldo could be absent for his nation's matches against Russia and Northern Ireland, according to Sky Sports News.

Portugal will take on Russia this Friday, which will then be followed by Northern Ireland on Tuesday.