Chelsea have been rocked by the news that Michael Essien faces several months on the sidelines after rupturing his anterior cruciate ligament.
Essien was stretchered off during Ghana's World Cup qualifying defeat to Libya on Sunday and is likely to miss at least six months of the new Premier League season after scans confirmed the severity of the injury.
The 25-year-old had only just returned to action after missing the opening weeks of the campaign with a calf problem.
"He (Essien) has ruptured his anterior cruciate ligament and will undergo an operation as soon as the majority of the swelling has subsided. We expect this to happen in a week's time," read a club statement.
"In the meantime, Michael has started rehab in the aqua-therapy centre at Cobham which will improve the condition of the knee prior to surgery.
"A return to full fitness is not expected for several months."
Chelsea boss Luiz Felipe Scolari now faces a major selection headache in midfield, with Michael Ballack and John Obi Mikel also currently sidelined.
The Blues may have face Manchester City on Saturday evening without Essien, Ballack, Mikel and fellow injury absentee Didier Drogba.
The Ivory Coast striker remains on the comeback trial following summer knee surgery.