Chelsea midfielder Mateo Kovacic has tested positive for coronavirus just a day after returning to training, manager Thomas Tuchel announced on Tuesday.
Kovacic tests positive for Covid in 'huge setback' for Chelsea
Chelsea's Croatian midfielder Mateo Kovacic
The 27-year-old has been out of action since late October and Tuchel had hoped to ease him back into action this week.
N'Golo Kante remains sidelined with a knee issue, while Jorginho has been forced to play through the pain of a back problem and will now miss Wednesday's Champions League trip to Zenit Saint Petersburg, leaving Tuchel short of defensive midfielders.
"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.
"But he tested positive today, for coronavirus, so he's quarantining for a few days. So it's a huge setback for him personally and for all of us."
The defending champions have already qualified for the knockout stages of the Champions League and will be confirmed as winners of Group H if they equal or better Juventus's result against Malmo this week.
Securing top spot could offer an easier knockout draw, but Tuchel will balance those interests with the need to keep his players fresh for Saturday's Premier League clash with Leeds at Stamford Bridge.
Tuchel, who is also missing defenders Trevoh Chalobah and Ben Chilwell, admitted the midfield injuries had taken a hefty toll, with Chelsea knocked off the top of the Premier League at the weekend after defeat at West Ham.
"The game tomorrow in isolation is not the problem in terms of being overloaded, it's more the game that comes up on Saturday," said the German manager.
"We will lose time to sleep, we will arrive back in the very early morning of Thursday," he added.
"And on Saturday we will face a very intense team of Leeds, who have prepared the whole week for this match.
"So long story, short message -- there's a huge chance that we rest some players and maybe more importantly we give some players the load who need the load, the minutes and rhythm, to put it another way."