Modric is reportedly keen to join the Spanish champions, but Tottenham will not let the Croatia international leave until their valuation is met.
Real captain Casillas did not want to be drawn too much into the club’s bid to sign Modric, but has backed Mourinho’s judgement when it comes to improving the squad.
“I know him,” the Spain goalkeeper told AS. “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 hoping that Real can reach the UEFA Champions League final this season after falling at the semi-final stage in the last two years.
He added: “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.”