Ghana midfielder Michael Essien has picked up a knee injury during Chelsea’s pre-season training, the English Premier League side said on Friday.

“Michael Essien suffered an injury to his right knee during training this week,” Chelsea, who finished runners up to English champions Manchester United last season, said in a statement on their official website.

"He will undergo tests over the next few days to determine the extent of that injury. Until we have the full results of those tests we will not be commenting further."

Sky Sports reported that Essien, who was plagued by injuries in recent season, might have ruptured cruciate ligaments, an injury which could sideline him for several months.

Chelsea’s new Portuguese boss Andre Villas Boas will be keeping his fingers crossed that the injury does not turn out to be that serious.

Essien pulled out of the 2010 African Cup of Nations after suffering a medial ligament injury. He consequently missed the World Cup few months later before going on an extended break from national duty to concentrate on his club commitments.

After an 18-month absence, the 28-year-old made his Ghana return in the 3-1 African Nations Cup qualifying victory over Congo last month.