Kroos played as Madrid cruised to a 4-1 win over 10-man Juventus in Cardiff on Saturday, becoming the first team in the Champions League era to retain their title.
Ronaldo scored two crucial strikes either side of Casemiro's deflected effort to ensure he finished as the competition's top scorer for a record fifth straight year, and sixth time overall.
"Everybody knows that Cristiano is very important," Kroos said to BT Sport.
"As a team we played very well in the quarter-finals, the semi-finals and the final - but you need a guy to score the goals and he did it again.
"I didn't expect that it was possible to defend this title, it is so difficult to win it once.
"To win it three times in four years means a lot and it is not only a good team on the pitch, but off the pitch, I am happy to be part of it.
"Juventus are a very good defensive side but we showed our quality."
Marco Asensio scored Madrid's fourth after coming on as a late substitute and the attacking midfielder was ecstatic to finish his campaign with a huge high.
"I'm really happy with how things have gone for me and the great year that we've had as a team," he said.