A Luis Suarez brace and Raphael Varane own goal saw Barca claim a 3-0 win at the Santiago Bernabeu on Wednesday, sealing a 4-1 aggregate success in the semi-finals.
Vinicius Junior, 18, had Madrid's best chances but was unable to convert, and they were punished in the second half.
Casemiro said there was no reason to discuss Juventus' Ronaldo once again, despite the former Madrid star's absence being felt by the European giants.
"We cannot talk about a player who is not here," Casemiro told reporters, via AS.
"A week ago we talked about [Karim] Benzema being the best nine, Vinicius, Lucas [Vazquez]...we cannot talk about Cristiano because he's not a Madrid player.
"We missed a goal, but the top players are doing everything very well, especially Karim, who is making his best season at Madrid."
Casemiro also refused to blame Vinicius for the loss with the Brazilian attempting five shots in the first half against Barca but failing to find the back of the net.
"Speaking only about him is unfair. He is daring, courageous and not afraid of anything," he said.
"His level is spectacular for his age."
Real Madrid coach Santiago Solari praised his side's efforts in the defeat declaring they applied themselves well but lacked the decisive touch.
With Barcelona through to a sixth straight Copa del Rey final, Solari believes falling at the semi-final stage isn't a failure for Madrid.
"We have been beaten with honour, but we're sad because this is a tough blow for us to take," Solari said.
"It isn't a failure because it was a semi-final and we won't be able to play in the final, so we will have to try again."