Blues midfielder Mateo Kovacic has tested positive for Covid-19 just a day after returning to training following a long absence due to a hamstring problem. Thomas Tuchel's outfit travel to face Zenit St Petersburg in Russia on Wednesday night in the Champions League aiming to win their group but will be without the Croatian. Chelsea will have been hopeful to have Kovacic fit and available when they return to top flight action on Saturday afternoon in the capital against the Whites at Stamford Bridge (3pm). The 27-year-old, though, is now set for a period of isolation which will deepen his side's current midfield problems. Tuchel already has N'Golo Kante and Jorginho out for their midweek action in Europe while full-back Ben Chilwell (knee) is waiting to see if he will play again this season. “We have bad news because Mateo was in training yesterday with a big smile, pure pleasure to have him back, and he tested positive today for corona and is isolating,” Tuchel told his pre-Zenit press conference. “He will be out for several days, which is a huge setback for him and me, and all of us. This is what reality is at the moment so we have to be ready to adapt and secure people’s health.” Kovacic has been absent for Chelsea since October 26 and is now set for a further two weeks out. Ross Barkley, Ruben Loftus-Cheek and loanee Saúl Ñíguez could provide cover should they be required in central midfield. Trevoh Chalobah has been battling a hamstring injury in recent days while Marcos Alonso and Kai Havertz have shaken off knocks for the Blues.