Tottenham midfielder Modric has been heavily linked with a move to the Bernabeu this summer, although so far Spurs have resisted all interest, insisting they will hold out for the price they want, believed to be in the region of £40million.
However, Real captain Casillas describes Modric as "a great player" and is happy to trust the judgement of boss Jose Mourinho.
"I know him," said the 31-year-old keeper. "He has been a great player ever since he signed for Tottenham. I faced him in the Euros and he had a very good season.
"If the manager thinks he ought to be then I am sure he will be at the level of this squad. It is not our responsibility to say anything more.
"The club and the manager are those that decide what is best for the team to move a step forward."
Meanwhile, Casillas is determined to claims Champions League glory this season, having gone out at the semi-final stage in each of the past two years.
He told AS: "Our goals are similar to those last year. We are hoping to move a step forward like we have in the other seasons since Mourinho has been with us.
"We want to win the league again, the Super Cup and then after two years of arriving at the doors of the Champions League final, we want to reach the final of Europe's main competition.
"I think we have extraordinary fans, a magnificent stadium and an incredible squad. With all of this it is clear that this team deserves a European Cup, but there are many teams fighting for this title.
"There are steps we have taken. For example, two years ago players like (Mesut) Ozil or (Sami) Khedira didn't have the experience they have now. The same applies with Karim (Benzema). But thinking about the 10th European Cup is not an obsession because we face the start of the league and the Super Cup first."