Michael Essien is to undergo tests this week to discover the severity of the knee injury that has ended his African Cup of Nations.
Chelsea are understood to be concerned the injury may be serious, with the initial diagnosis showing that he has suffered a partially torn medial collateral ligament in his left knee and damage to his meniscus.
Essien suffered the injury during training with Ghana and could be out for around six weeks, with the influential midfielder having now returned to London.
Having only just returned from a hamstring injury Essien admits this latest problem has come as a hammer blow.
"I am so unhappy because I didn't think it was a grave blow," he is quoted as saying in The Sun.
Ghana team-mate Anthony Annan admits the whole squad were immediately concerned when Essien went down unchallenged in training.
"Michael was turning to make a pass when he just went down," said Annan.
"There was no contact from any other player, but those around him say they heard a click and the physios rushed over."
Matthew Amoah believes the injury could not have happened to a more important player as Ghana bid for glory in Angola.
"This is a big blow because Michael is so important to our team," said the striker.
"We've lost the motor in our midfield, and this is very bad."