Chelsea boss Thomas Tuchel has labelled Mateo Kovacic's absence from training as a 'huge setback'.
The Blues midfielder has recently been on the sidelines due to a hamstring injury and hasn't featured for the west London side since their Carabao Cup victory against Southampton at the end of October.
However, he has suffered another setback despite the Croatian being on the verge of returning to first team action.
Speaking to the media ahead of his side's Champions League tie against Zenit on Wednesday evening, Tuchel provided an update on the midfielder's situation as they enter a busy festive period.
“We have bad news. Mateo was in training yesterday with a big smile and a pure pleasure to have him back.
"He got tested positive today for Corona so he is isolating. Current headache. He is out for another several days, a huge setback for him personally and for all of us.”
Kovacic's absence from the team means Chelsea only have two fit central midfielders in Ruben Loftus-Cheek and Saul Niguez, with N'Golo Kante and Jorginho also injured.
Chelsea's December schedule sees them play nine games in three different competitions.
They have already won and lost at Watford and West Ham respectively, with Wednesday's match against Zenit being their third of the month.
The Blues will have another five games to play in the Premier League before the turn of the New Year as well as a Carabao Cup quarter-final tie away at Brentford, whom they beat 1-0 earlier in the season.