The 34-year-old was benched for Primera Division matches under Carlo Ancelotti last season and criticism over his performances was one of themes of the club's below-par campaign.
Casillas has now become one of the first departures since Benitez joined from Napoli – joining Porto on a two-year contract – but the former Valencia boss paid tribute to the club and national hero.
"Iker is a Real Madrid living legend and the best goalkeeper in the club's history," he told Madrid's official website. "We all know how important it has always been.
"However, he has decided to go to another team and leave. As a coach, I have to respect that decision and wish him all the luck in the world.
"I think he will stay in the heart of all the Madrid fans and I wish him success. I'm sure all will go well at the great club and city of Porto. There's little more I can say other than that he has my total respect and admiration."
Benitez also gave his verdict on Cristiano Ronaldo, who beat his best-ever goal tally in a single season in 2014-15 despite Madrid's failures in La Liga, Copa del Rey and the Champions League.
"Cristiano is a fundamental player to us and the fact he is here in pre-season training with the whole group is very satisfying," the new boss added.
"Once I get to know him more and understand how he thinks, then I will be able to speak more openly about how he will work in the group dynamic. I'll get to know him better, he'll perform better and he'll take this team to its full potential."
Real Madrid begin their coming campaign away to newly promoted Sporting Gijon.