The Croatia international has been in impressive form for the Blues during the 2021-22 campaign, contributing one goal and five assists in 13 appearances in all competitions.
However, the 27-year-old is now facing a period on the sidelines with the issue that he suffered "in the last minute of training" on Thursday.
"We had very sad news on Thursday. He injured himself in the last minute of training," Tuchel told reporters. "He suffered a hamstring injury and will be out for some weeks actually, next week for sure and then comes the international break.
"Hopefully after the international break he will be back. He was doing a little bit too much in training so it's my fault a little bit."
Kovacic, who has a contract at Stamford Bridge until June 2024, has made 153 appearances for the Blues since arriving from Spanish giants Real Madrid. body check tags ::