Influential defender Lees has missed Wednesday’s last four fixtures after suffering a hamstring tear in their loss at Stoke City earlier this month.
Lees, a key figure in the Owls equalling a club-record of 17 league clean sheets last season, continues to receive treatment from Wednesday’s medical team.
His rehabilitation is going according to plan but The Star understands the former Leeds United player may not be ready to return to action until mid-January.
Carvalhal said: “Tom is working to get back as soon possible. We don’t know yet when he will be back but he is improving day by day. He is getting better.”
Lees was the only person to start the opening 23 matches of Carvalhal’s reign, with the Portuguese generally favouring a squad rotation policy.
Centre half options are currently few and far between for Carvalhal as Lees, Michael Turner and Vicent Sasso (both knee) are all on the sidelines.
Midfield trio Alex Lopez (thigh), Marco Matias (hernia operation) and Filipe Melo (knee) are also ruled out.
Carvalhal, who has rubbished speculation linking him with the vacant managerial job at Swansea City, added: “We want all the players to come back as soon as possible but we have to be careful.”