Essien ruled out for up to six months
The club confirmed on Friday that the Ghanaian had damaged his knee, but stopped short of commenting on its severity, instead stating that they would wait for Essien to be assessed.
However, the results were not positive, and the midfielder has since undergone surgery to repair the damage.
"Following injury to his right knee in training last week Michael Essien had surgery today to repair a ruptured anterior cruciate ligament and meniscus," read a statement on the club's official website.
"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."
Essien damaged his anterior cruciate ligament while on international duty in September 2008, ruling him out until March 2009, while he also missed the 2010 World Cup through injury.
More to follow.