Mateo Kovacic testing positive for Covid-19 is a "huge setback" for Chelsea, says manager Thomas Tuchel.
The Croatian midfielder had returned to training on Monday after being out since October with a hamstring problem, but tested positive on Tuesday.
"He was in training yesterday and with a big smile and it was a pure pleasure to have him back," said Tuchel.
"But he tested positive today, for coronavirus, so he's quarantining for a few days."
Tuchel, speaking before Wednesday's Champions League trip to Zenit (17:45 GMT), added: "It's a huge setback for him personally and for all of us."
Chelsea, last season's tournament winners, have already qualified for the Champions League last 16 and a win in St Petersburg will ensure they progress as group winners.
Tuchel had hoped to ease Kovacic, 27, back into action this week with N'Golo Kante sidelined with a knee issue and Jorginho struggling with a back niggle which will keep him out of Wednesday's game.
Asked if the resurgence of Covid-19 has caused extra concerns at Chelsea generally, Tuchel added: "Yes, I was aware that this can happen.
"I'm not happy not only as a manager but also as a person. But this is the reality at the moment, we have to face it to be ready to adapt and to do the best thing possible to avoid transmission."
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