Chelsea have confirmed that Michael Essien could require up to six months of rehabilitation after undergoing knee surgery.
The injury-plagued Ghanaian ruptured anterior cruciate ligaments and meniscus in his right knee in pre-season training last week.
Essien, who has suffered cruciate and medial ligament injuries previously in his career, has since had an operation and may not return until January 2012.
A short statement on Chelsea's official website read: "Following 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."
The 28-year-old was unavailable for a large chunk of the 2008/09 season due to cruciate knee ligament damage.
Essien then suffered a medial ligament injury at the African Nations Cup last year and missed the 2010 World Cup as a consequence.
Chelsea only retuned to pre-season training last week and new boss Andre Villas-Boas hopes that the midfielder will make a swift recovery.
Villas-Boas said: "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."