Chelsea midfielder Michael Essien has been ruled out for "several months" after rupturing his anterior cruciate ligament, the club has announced.The 25-year-old suffered the injury during Ghana's World Cup qualifier against Libya in Tripoli on Friday.Essien will undergo surgery once the swelling around his knee has subsided.England striker Michael Owen picked up the same injury during the 2006 World Cup and was ruled out of competitive action for 10 months.Essien has been a regular for Chelsea since his £24m move from Lyon in 2005, scoring 14 goals in 132 appearances.A Chelsea statement read: "Michael 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."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."The loss of the combative Ghanaian for such a long period is a major blow to Chelsea, who have no like-for-like replacement available and will not be able to buy any fresh faces before the transfer window re-opens in January.Essien had landed awkwardly during the qualifier against Libya and was immediately taken to hospital for scans, with the diagnosis now confirmed by Chelsea's own medical staff.Luiz Felipe Scolari's side, who opened their Premier League campaign with two wins and a draw, are next in action away to Manchester City on Saturday before facing Bordeaux in the Champions League and champions Manchester United in the league on 21 September.Source: BBC