Chelsea have confirmed Michael Essien's latest knee injury will keep him on the sidelines for up to six months.
The Ghana midfielder damaged ligaments in his right knee during a training session last week, the third serious knee blow of his career, and has undergone an operation.
A statement on Chelsea's official website read: "Following an injury to his right knee in training last week, Michael Essien had surgery today to repair a ruptured anterior cruciate ligament and meniscus. The surgery was successful and Michael's full recovery could take up to six months."
Chelsea manager Andre Villas-Boas added: "Everyone at Chelsea wishes Michael well for his recovery. His team-mates and all the staff and I look forward to helping him return to playing as soon as possible."
Meanwhile, Villas-Boas has revealed his desire for Chelsea to win with style this season.
The new Chelsea manager moved to Stamford Bridge from Porto, who scored a remarkable 145 goals in 58 games during their title-winning campaign last year.
Villas-Boas worked at Chelsea under Jose Mourinho, whose more pragmatic style reportedly frustrated the club's owner Roman Abramovich.
But the 33-year-old believes the key to success for Chelsea, who are chasing their first Champions League crown, will be to take the shackles off.
"You free them (the players) from the limitations that they might have in a different kind of system but, in the end, you make them believe in their qualities and this is the most important thing," Villas-Boas told Chelsea magazine.
"The club hasn't stopped in time since I left, it has evolved in various ways but to have the kind of empathy that I have with the people is decisive. It goes from top to bottom and it's with people who meant a lot to me when I was here.