2. Chelsea doesn’t have front foot defenders
Chelsea don’t have defenders that look to press higher and win the ball. They don’t have centre backs who want to start offensive moves from the back. Generally Cahill and Terry are more suited to drop deep and absorb pressure, meaning Chelsea usually play deep when in times of crises. Chelsea need players who are mobile and aggressive in their defending. Cahill and Terry are aggressive defenders, but their lack of sheer defensive pace or strength means that they lack in the area when the team opens up to play.
Generally when the team plays attacking football, the defensive midfielder drops deeper to collect the ball and help the team to start offensive moves from the back with better technique and vision. Here Matic is doing the job for Chelsea which otherwise a ball playing center back would have done. The Serb is under immense pressure game after game to cover for Ivanovic as well as Terry, with both failing to track back quickly enough, while his partner Cesc Fabregas is generally an attacking midfielder.