The erratic Luiz has borne the brunt of the criticism amid Chelsea's inability to keep clean sheets.
Cahill said: "David Luiz is one of the most comfortable players on the football as a centre-half I've seen. He's got great ability.
"You don't play for a top club and captain Brazil if you're not a top footballer. That's what he is.
"John (Terry) is a different type of player. They're both fantastic. Defensively, it's been two halves this season.
"Early on, we kept six clean sheets and had one of the best defences in the league.
"Of late it's not been the case. It's the same personnel, the same team, so we just have to get back to the way we started the campaign."