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 capital 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, which confirmed a 5-0 aggregate progression into the final.
And Casillas slammed the fan who hurled the object towards the prolific attacker, claiming that his 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 his club's official 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 over the semi-final, remarking: "The first leg was key - winning 3-0 made the second leg more straightforward.
"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."