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."