Chelsea midfielder N'Golo Kante has tested positive for coronavirus and will miss Wednesday's Champions League tie with Juventus.
The 30-year-old France international will isolate for 10 days.
He will not feature against Southampton in the Premier League on Saturday and is also likely to miss France's Nations League game with Belgium on 7 October.
Blues boss Thomas Tuchel said he did not know how many of his players are vaccinated against Covid-19.
"We are a reflection of society, the players are adults and they have a free choice," Tuchel told a news conference on Tuesday.
"We should accept it. This is more or less all I can say. I know the situation is far from over. It makes you very aware it is not over."
When asked if players should lead by example and get the vaccine, Tuchel said: "Do I have the right to say it? I'm not so sure.
"It is a serious question, vaccinations seems to be a proper protection - I am vaccinated but I don't see myself in the position to speak out on recommendations."
Chelsea will also be without Reece James, Christian Pulisic and Mason Mount against Juventus.
Defender James injured his ankle against Manchester City on Saturday, while Mount and Pulisic are missing with longer-standing injuries.
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