Zhirkov hurt his right knee in pre-season against AC Milan, though it is thought the problem may have been the result of playing for years on the artificial pitch at Moscow's Luzhniki Stadium.
He has since felt pain in both knees and suffered swelling after playing for less than 20 minutes as a substitute against Atletico Madrid in the Champions League last month.
Chelsea doctors have been so baffled that they have consulted experts in France and Germany while working on a number of theories as to the source of the problem.
"The English doctors see my problems as a reaction of the body to drastic changes: in the climate, lifestyle, training work and in everything else," he said. "And no one can tell me what the medical predictions of my rehabilitation are.
"The British physicians even consulted with German specialists regarding me. They sent them my X-rays together with the detailed description of the problem but, as far as I understand, they failed to diagnose the problem.
"I try to remain optimistic: many players have had such difficulties. I am not the first, not the last. It is a pity that the problems coincided with my move to a new club. But if I keep being distressed the situation will not change. I must work and believe that everything is going to straighten out soon.