Kalou, who missed two easy chances early on, scored once in each half. He was supported by Frank Lampard's first strike in open play this season and an own goal by Luis Perea.
"After not playing a couple of games, to come back into the team it was important for me to get confidence back by scoring," said the Ivory Coast international. "All the boys were saying after I missed the first opportunity, 'Don't worry about it, it will come', and I'm happy it came in the end."
He added: "It was very important to win that game because it gave us nine points."
The win left Chelsea in firm control of Group D. They are eight points ahead of Atletico and three in front of second-placed Porto.
Kalou put Chelsea on the way to victory when he converted a cross from Ashley Cole in the 41st minute.
He added the second six minutes after the restart before Lampard fired home his first goal in 10 games and Perea headed into his own net in the closing seconds.
"It's good to get confidence back by winning and everyone gave his best and we have the result in the end," he added. "After the disappointment of last week [against Villa] we had the opportunity to show it was a mistake."
Blues boss Carlo Ancelotti was delighted with Kalou's performance, which came just weeks after the player signed a new three-year deal with the club.
"Kalou is an important player for us because, when we are without Didier Drogba, he can play and play well," said the Italian. "For him, it was very important to score. Also for us. He played very well for us."