Chelsea will know the extent of Michael Essien's injury by Friday.
The Ghana midfielder is scheduled to arrive back at the club's Cobham training ground this week in order for Chelsea's own medical staff to assess his knee injury.
Essien was forced out of Ghana's African Nations Cup campaign after hurting his knee during training.
Essien, 27, could be out of action for at least a month according to initial scans undertaken in Luanda.
They are understood to show that he has suffered a partially torn medial collateral ligament in his left knee and damage to his meniscus.
A spokesperson for Chelsea confirmed: "We are aware of the injury to Michael and have been in contact with the Ghanaian national team.
"We will fully assess the injury on Michael's return to us later this week. Until then we are unable to comment further."
Ghana's players feared the worst when Essien, who has only just recovered from a hamstring problem, went down during a training session.
"This is a big blow because Michael is so important to our team," said striker Matthew Amoah.
"We've lost the motor in our midfield, and this is very bad."
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