The deadly duo both found the back of the net as the Blues made it four Premier League matches on the bounce.
And Cahill believes Costa and Hazard provide that extra quality the top teams need to mount a title challenge.
He told the Evening Standard: “Yes. They are playing fantastic. It is important to have big players who can change a game.
“It’s important we have game-changers like Eden [Hazard] and Diego [Costa]”
Chelsea ace Gary Cahill
“You could see against Southampton we were trying hard to keep a clean sheet. We worked hard on keeping the shape and were 1-0 up.
“It is very tight when you’re just a goal up but then all of a sudden Diego turns and whips it into the top corner.
“That is sometimes the difference in players at this level, players we are blessed to have at this club.
“It is not just those two, though, we have many at this club. But it’s important we have game-changers like Eden and Diego.”
Next up for the Blues is a home clash against Everton on Saturday.