Essien missed much of the 2008/2009 campaign because of a anterior cruciate ligament problem suffered while on international duty and to lose the 28-year-old once again would be a bitter blow for player and club.

“Michael Essien suffered an injury to his right knee during training this week,” read a statement on Chelsea’s official website, www.chelseafc.com.

“He will undergo tests over the next few days to determine the extent of that injury. Until we have the full results of those tests we will not be commenting further.”

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.

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.