The Blues skipper picked up a knee injury in last weekend's clash with Liverpool that will keep him out until early December.
Terry was forced to sit out yesterday as Chelsea slipped to a surprise defeat against West Bromwich Albion.
Di Matteo believes that the team's confidence in their ability to keep clean sheets has diminished during Terry's absence.
He told Sky Sports News: "Clean sheets certainly give you a chance to win games and we certainly have to improve on that side.
"Defensively we just weren't good enough. Of course we miss John. He is an important player for us.
"He gives us a lot of confidence at the back and in general to the team. It's a shame that he is injured, but we will have to do without him until he comes back and improve."
Terry will continue his enforced spell on the sidelines on Tuesday when Chelsea take on Serie A holders Juventus in a crucial Champions League tie.