The positive test for Covid-19 will cause Mateo to miss the game against Zenit.
The news was described as a huge setback by the head coach at the pre-game news conference.
On Monday, after he had returned to training with a strained groin, he returned a positive coronaviruses test on Tuesday.
The Blues had hoped to get him back in action this week, but he was not ready.
N'Golo Kante is still out with a knee issue, while Jorginho has a back problem and will miss Wednesday's game.
"We have bad news because Mateo was in training yesterday and with a big smile and it was a pure pleasure to have him back," said Tuchel.
He's quarantining for a few days because he tested positive for coronaviruses. It's a huge blow for him personally and for all of us.
Tuchel said that he was aware that the resurgence of coronaviruses could happen to him.
I'm not happy both as a manager and a person. We have to face it and do the best we can to avoid transmission.
Tuchel has built a machine.
Sergei Semak praised Thomas Tuchel's "well-oiled" system.
If the Blues equal or better the result of the game between the two teams, they will win the group.
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Manchester City traveled toWatford, while West Ham hosted Chelsea, and Wolves faced off againstLiverpool.
The quality of craftsmanship that manager Tuchel has put into refining his tactical set-up is something that Semak knows.
"You're talking about individual players but I would like to point out the importance of the system in general, they are like a German machine, well-oiled," said Semak.
Every person on the pitch knows how to do it.
I would like to concentrate on the team and not talk about individual qualities.
Tuchel is upset about big mistakes.
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The coach of the team says that they have made too many big mistakes in recent games.
After a 2-0 defeat at West Ham, Thomas Tuchel was left to rue a lot of "big mistakes".
The leaders of the league were aware that a victory at the London Stadium would move them four points clear of their rivals and they were on their way to the capital.
West Ham's three goals, including Arthur Masuaku's freakish winner, were the result of an error-strewn performance by goalkeeper Edouard Mendy.
"I didn't see a dip in our performance against Manchester United or today, it was enough to win the game," Tuchel said.
"We gave easy goals away, this is unusual - big mistakes that lead to big chances and easy goals." We did this against United, and today.
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Tuchel said that Mendy is going through a rough period and is suffering from a loss of confidence.
We aren't good enough in the details at the moment. I can only repeat myself in this game. There's a chance we win this game if we don't do big mistakes.
Thetitle race is not interesting at the moment, what is interesting is the next 90 minutes. We need to have the focus right, the attitude right, and the details right in order to take care of the details.
It's impossible to do these kinds of mistakes if you want to protect yourself. We have done these three times in a row.
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