Antonio Conte decided to allow the 29-year-old Serbia international to join their Premier League rivals for a fee in the region of £40 million, with Matic having been deemed surplus to requirements.
The champions still have N'Golo Kante and have also signed Tiemoue Bakayoko from Monaco in the summer transfer window, but Cahill thinks the club will feel the loss of former Benfica midfielder Matic.
When asked about the Serb, Cahill told Sky Sports: "Matic is a fantastic player, a real top player. I had the benefit of realising how good he is when he's playing just in front of you, on and off the ball.
"He's going to be a huge loss, there's no doubt about that. And for us it's important because it can be very offensive this formation.
"People look at it as a five [in defence] but really it's a three a lot of the time. The two wing-backs are pushed very high and we attack with numbers, so for us at the back in terms of balance you have to have some sort of protection and they gave us that last season."
The Blues begin the defence of their title with a home fixture against Burnley on Saturday as the Premier League gets back underway for the 2017-18 campaign.