Ronaldo has been at the middle of intense gossip about his future after revealing following Madrid's home win over Granada earlier this month that he did not celebrate his two goals in the match because he was "sad" for professional reasons, but without elaborating.
The Spanish media were quick to suggest the 27-year-old, who has scored 150 goals in 149 matches for Real, was eager to leave the Bernabeu, while his desire for a new improved contract was also mooted as a possible reason for Ronaldo's discontent.
The Portugal star has twice taken to Facebook to deny the contract suggestion but that has only led to more talk about what has caused his misery.
Casillas believes it is time to refocus on on-field matters and urged fans to simply enjoy Ronaldo's talents.
He said: "For Madrid, both as a player and a fan, it is a real pleasure to have Cristiano. And for Cristiano, it is a luxury to wear the shirt of the best club in the world.
"Everyone is better off. Cristiano is happy to stay in Madrid and the fans are happy to have Cristiano. Madrid's fans are demanding, but they are with everyone.
"Now we have to think of Saturday because we have a very difficult match in Seville. We have to get the three points. If we don't do the right things it will endanger our chances in the league."
Casillas denied reports of a showdown meeting between Ronaldo and Madrid boss Jose Mourinho this week, adding: "I saw Ronaldo (on Wednesday) and he was happy. We were discussing the results of the international teams.
"I arrived at training at 4.15pm and there was no meeting, there was nothing out of the ordinary. Cristiano was normal. We have to be focused on the Sevilla match, we cannot drop more points, we have to win."
Madrid have made a slow start to the defence of their La Liga title having won just one of their opening three games. That victory came against Granada in their last outing but Mourinho was still not satisfied, afterwards questioning his side's hunger.
Casillas said: "The coach is demanding.
"We want to recover the identity we had last season when we won a competitive league."
The Ramon Sanchez Pizjuan has been a happy hunting ground for Madrid in recent years with their previous two visits ending in 6-2 victories - Ronaldo scoring seven of Madrid's 12 goals.
But they will have to keep a close eye on Sevilla's Alvaro Negredo.
Negredo was in the title-winning Spain squad at Euro 2012 but was left out for the opening World Cup qualifier against Georgia in midweek.
The 27-year-old who came through the ranks at Real Madrid but failed to make a single first-team appearance in the Primera Division had to then watch as his replacement Roberto Soldado scored the only goal in Tbilisi.
He told Radio Marca: "I must continue working hard, The coach had to decide and opted for Soldado.
"If I'm not in the Spain squad it is because the coach did not like something. There is lots of competition in that position."
Sevilla have yet to lose in their first three games but have won only once - an opening day 2-1 victory at home to Getafe.