Chelsea is likely to be without Michael Essien for up to six months after he underwent knee surgery on Monday.
The Ghana midfielder injured his right knee in preseason training last week.
"Essien had surgery to repair a ruptured anterior cruciate ligament and meniscus," Chelsea said in a statement. "The surgery was successful and Michael's full recovery could take up to six months."
Chelsea's Premier League season begins on Aug. 14 at Stoke.
"Everyone at Chelsea wishes Michael well for his recovery," Chelsea manager Andre Villas-Boas said. "His teammates and all the staff and I look forward to helping him return to playing as soon as possible."
The 28-year-old Essien was sidelined for six months in 2010 because of an injury to the same knee, ruling him out of Ghana's World Cup campaign.
He previously injured his anterior cruciate in his left knee in September 2008 and was out until the following March.