Chelsea midfielder N'Golo Kante has tested positive for Covid-19 and has gone into self-isolation.
Manager Thomas Tuchel confirmed Kante must "quarantine and follow the government protocols."
Tuchel said Kante was not at training on Tuesday as Chelsea prepared for their Champions League Group H match against Juventus on Wednesday.
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The 30-year-old will also miss Saturday's Premier League clash with Southampton and his participation in France's Nations League clash with Belgium on October 7 could also be in doubt.
Tuchel said Christian Pulisic, Mason Mount and Reece James will also be unavailable against Juventus.
Pulisic and Mount have been out with prior injuries, while James twisted his ankle and was withdrawn after 29 minutes of the 1-0 home defeat to Manchester City on Saturday.
Tuchel: Do I have the right to tell my players to get vaccinated?
Last week, Sky Sports News reported a new push to increase the number of coronavirus vaccinations among elite players was underway within Premier League clubs, after concerns were raised about the poor take-up levels.
Tuchel said he did not know what percentage of his players had received the vaccine and argued it was not his responsibility to make that decision for them.
"We are also only a reflection of society," he said. "the players are adults and have free choice if they want to get vaccinated or not, and I think we should accept it.
"I know the situation is far from over and all the time somebody gets [the virus] here from us and our group, it makes you very aware that it’s not over."
"It would be easy to say yes [Chelsea players should all be jabbed] now and get the applause from a lot of people but at the same time, do I have the right to say it? I’m not so sure.
"I can make the decision for myself and everybody else needs to reflect about it and take the risk or not.
"It’s a serious question and vaccination seems to be a proper protection. I am vaccinated - I took the decision for myself. But I don’t really see myself in the position to speak out proper recommendations.
"I think that would go too far. I am a football coach, I am not a 100 per cent expert in this and I would leave it up to them.
"Everybody is adult enough and we live in a free country, a free society which is a good thing, so people can make their choices.
"We don’t like to lose players but we are not angry with N’Golo, by far not. I am concerned about him, I am worried about him like I am worried about my injured players who receive a kick in the game.
"Now we will stay in close touch and take care of them and bring them back as soon as possible."
Juventus duo Paulo Dybala and Alvaro Morata have been ruled out of the Champions League group-stage clash with Chelsea this week because of injuries.
Dybala suffered a thigh problem in Sunday's Serie A win over Sampdoria - withdrawn in tears after just 22 minutes and having scored his side's opener, while Morata also came off with eight minutes of normal time remaining.
Manager Massimiliano Allegri confirmed both players will miss the tie at Juventus Stadium and the subsequent derby with Torino, but he hopes to have them back after the next international break in early October.