The central defender suffered a knee injury during the Blues' 1-1 draw with Liverpool at Stamford Bridge in the Premier League on Sunday.
"With JT we're looking at approximately three weeks out," said Di Matteo. "Fortunately it's not as bad as we first thought.
"It's an MCL (medial cruciate ligament) strain."
Chelsea travel to West Bromwich Albion in the Premier League on Saturday.