The European champions lost their first competitive match since Ronaldo's £100m switch to Juventus, going down 4-2 after extra time at the hands of Atletico Madrid in the UEFA Super Cup.
Casemiro insists that losing the five-time Ballon d'Or winner cannot be used as an excuse, however, and the Brazil international was also quick to defend boss Julen Lopetegui following the loss in Tallinn.
"Sure, we cannot cover things, Cristiano showed here that he has been the best in the world," he told reporters. "But it is already past, we cannot keep thinking about him.
"We have to talk about the players we have. We want to make a stronger, more solid team, but Cristiano is already in the past. It's still the beginning of everything.
"Julen is doing a great job, trying to maintain the illusion, which is not easy. We have won four Champions Leagues in the last five years.
"We have won almost everything, he is doing a great job maintaining the players' illusion of winning. It's still too early to talk about Julen. We cannot cover Julen's work by tonight's result."
Atletico have now won the UEFA Super Cup three times in the last eight years.