Terry, who only returned from his four-game domestic suspension in last weekend's 1-1 draw with Liverpool, suffered a knee injury after a collision with Luis Suarez.
The former England man was stretchered off after the clash and it was feared Terry had sustained a serious injury.
However, scans have revealed the extent of the injury is not too serious and it will not rule out Terry for a lengthy period.
"With JT, we're looking at approximately three weeks," said Di Matteo.
"Fortunately, it's not as bad as we first thought. It's an MCL (medial cruciate ligament) strain.
"On the one hand, we're obviously pleased it's not as bad as first feared.
"On the other hand, we're disappointed we're losing him again after he just got back.
"It's a crucial period of the season for us, too. So that's a shame.
"It will depend on the progress of his rehab. We'll do everything to speed it up, but it depends how he reacts as well."
Meanwhile,Frank Lampard is still sidelined with a calf injury which has kept him out for three and a half weeks.
Di Matteo said: "We have to be careful with his injury because we don't want to take any risks.
"We want to make sure he's 100 per cent sure to be ready when we bring him back. He's not far off."