Chelsea have confirmed that midfielder Michael Essien has sustained a knee injury in pre-season training.
Reports had emerged from the Cobham training ground of last season's Premier League runners-up that the Ghanaian had damaged cruciate ligaments on the first day of full training.
Chelsea have not commented on the extent of the injury, as they await the results of further tests, but have verified that Essien is injured.
A statement on the club's official website read: "Michael Essien suffered an injury to his right knee during training this week.
"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."
Essien has been troubled by knee injuries in the past and it has been suggested that the problems have decreased his impact as a player.
The 28-year-old was unavailable for a large chunk of the 2008/09 season due to cruciate knee ligament damage.
Essien then suffered a medial ligament injury at the African Nations Cup last year and missed the 2010 World Cup as a consequence.
It had been suggested earlier in the summer that Essien could be sold and Inter Milan were linked with a bid.