The goalkeeper is saddened by both the fact that his team-mate had been struck with a missile and that a Rojiblancos fan had smeared the club's good name
The Portugal international was hit in the head when he was on his way to the dressing-room at half-time of los Blancos' 2-0 win (5-0 on aggregate).
Casillas has now slammed the fan who hurled the object towards the prolific attacker, claiming the assailant did more damage to his own club than Ronaldo.
"I think the consequences could be serious because it could have hurt a player," the Spain international told the official Madrid website.
"There is always someone who tries to smear the image of a club like Atletico and it doesn’t reflect how the majority of their fans are."
Casillas then went on to voice his satisfaction with Madrid's performance in the semi-final tie against their city rivals.
"The first leg was key, winning 3-0 made the second leg more straightforward," the shot-stopper acknowledged.
"We got two quick penalties and maybe that meant that the match wasn’t so exciting. We did our job well and deserve to be in the final."
Madrid will meet either Real Sociedad or Barcelona in the Copa del Rey final, with the Catalans 2-0 up going into their second-leg meeting with their Basque rivals in San Sebastian on Wednesday night.