Chelsea have announced that John Terry is to undergo an exploratory operation to determine the extent of his knee injury.
Andre Villas-Boas had been hopeful that Terry would be fit to return to the side for Tuesday's Champions League tie at Napoli, but after undergoing a fitness test on Monday it emerged that the problem was worse than feared.
Reports have suggested that Terry could be absent for up to two months, but Chelsea have announced that further tests will be required before a definitive prognosis can be offered.
The statement read: "John has tried hard to be ready for tonight's game but it was clear after training last night that it was not going to be possible.
"He will undergo an exploratory arthroscopy in the next day or so, after which the club's medical team will be better placed to advise on a prognosis."
Villas-Boas had said ahead of the Napoli game that he was hopeful the injury would not be as bad as feared.
"Hopefully John Terry will be out for less than six weeks," he told Sky Sports News. "They are cleaning the joint and hopefully it will be less."