The 25-year-old suffered the problem while on international duty with Ghana and, after an examination by the Premier League club's own medical staff, is set to have surgery next week.

Essien had already missed the opening match of the season, and is now set for an extensive spell on the sidelines - with a return not expected until well into the new year.

The loss of Essien is a severe blow for the Blues, 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.

Nigerian John Obi Mikel is battling to regain fitness for Saturday's trip to Manchester City following a leg problem, while Michael Ballack remains troubled by foot injury which made him unavailable for international duty with Germany.

Scolari will be hoping for more positive news when Frank Lampard returns from Croatia, a dead leg picked up in his last club appearance having at one stage been a worry for the England midfielder.

Another boost for the Brazilian coach could be the return of striker Didier Drogba from a knee injury in time to bolster the attack for the trip to Eastlands - where Scolari is likely to face off against former transfer target Robinho.

Scolari will certainly need all the men available heading into a tough run of fixtures, which also include the start of Chelsea's European campaign at home to Bordeaux and Stamford Bridge date with champions Manchester United on September 21.

The loss of Essien, though, will have left a big hole at the heart of Scolari's first choice XI - and the prognosis is not for a quick recovery.

A club 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."

Chelsea, meanwhile, could be set for a raid on their backroom staff should former Blues midfielder Gianfranco Zola be appointed as the new West Ham manager.

Italian Zola - who was a big favourite at Stamford Bridge during his playing days - is reported to be the front runner for the Upton Park hot seat and keen to bring in current Chelsea coach Steve Clarke as his number two.