Essien will have medical tests over the weekend to establish the full extent of the problem but officials at Stamford Bridge are worried the Ghana midfielder suffered yet another anterior cruciate ligament injury in training on Thursday.
Blues Brothers? Modric (right) could now join Luiz at Chelsea following fresh fears over injury-stricken Essien
It is a massive setback for new manager Andre Villas-Boas but, Sportsmail understands, it is something medical staff at the club feared might happen because Essien is one of those sportsmen whose over-developed? leg muscles put too much strain on? the knees.
Tennis superstar Rafael Nadal struggles with the same problem. He was denied the opportunity to defend his Wimbledon title in 2009 because of knee trouble and had to slim down to ease the stress on his joints. He has also required injections to relieve the problem.
Essien, 28, was taken immediately to hospital on Thursday and this looks like a third major knee injury. There are fears the situation could bring a premature end to his fine career.
He has been a key member of? Chelsea's side since joining from French side Lyon for ?24million in 2005, with the team significantly more successful when the player nicknamed 'The Train' is on the pitch.
But in September 2008, just two months after signing a new five-year contract, Essien suffered cruciate damage that put him on the sidelines for six months and he had medial ligament damage less than a year later, forcing him to miss the 2010 World Cup.
Head in hands: Essien could face lengthy spell on the sidelines after suffering a major knee injury during pre-season training
Plans were being drawn up to make landing another midfielder a priority for Villas-Boas.
Chelsea have noted Daniel Levy's determination to keep Modric at Tottenham but they will table a second offer in excess of their initial ?22m bid.
The key is the relationship between Modric, 25, and his chairman. The Croat firmly believes a verbal agreement to let him go should a suitable offer be received is in place and the situation could force Levy to reconsider the club's position if their midfielder continues? to protest.
If Chelsea think their pursuit of Modric is going to fail, they will have to look at other players.
Meanwhile, Chelsea have appointed Michael Emenalo as their technical director.
Emenalo, the former assistant first-team coach who replaced Ray Wilkins, will oversee a scouting network and youth development programmes, as well as supporting new manager Andre Villas-Boas.
Big say: Michael Emenalo has been named as Chelsea's new technical director
The 45-year-old was a surprise choice to succeed Wilkins in November because of his lack of experience at the highest level.
While Villas-Boas will have the final say on new signings, Emenalo expects to have an input into any transfers.
'The final decisions about who comes in and who doesn't will be the manager's to make, but to arrive at that stage there will be a lot of working together,' he said.
'I would imagine I will be part of that process of working together to make that happen.'
Chelsea have also promoted Dermot Drummy from youth-team to reserve manager, with Adrian Viveash taking his place in his previous position.
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Source: Daily Mail
Source: Daily Mail