The pair are said to have fallen out with each other, with the Portuguese recently claiming that he wished he had signed Diego Lopez earlier than this January.
"From this distance you do not understand the situation, given what Iker has always represented and what he still represents now," ESPN quotes Iniesta as saying.
"Not just that he is not playing, but the things that you hear. It is a difficult situation. Diego Lopez is playing very well, and is a great player. But I am not talking about the sporting situation, but the things that you hear."
Iniesta and Casillas are teammates for Spain at international level.