The Spain international feels his Madrid team-mate's performances merited continuing as the club's first-choice goalkeeper
The 32-year-old missed a large part of the second half of the 2012-13 campaign due to injury, but was widely expected to be reinstated between the sticks once he had regained full match fitness again.
However, Mourinho opted to stick with Lopez instead and Casillas had to settle for a spot on the bench, leading to a conflict between the Spain international and the controversial coach.
"Diego Lopez was superb and he deserved to continue playing. He had a brilliant season," Casillas told Marca.
"He is a fantastic person and a great goalkeeper."
Casillas has made over 650 official appearances for Madrid since making his first team debut in 1999-2000.