The Spanish winger may have some explaining to do when he returns to the Etihad after the international break after recommending his compatriot to replace Joe Hart.
"[Casillas] would be a great signing for City," Navas told Movistar.
"You always want great players in your team because they can be so important. We are not suddenly thinking this about Iker though. We all know what a great keeper he is and how vital he is to his team.
Navas is settling into life in England having previously undergone councilling for severe home sickness that even prevented him from travelling with the Spain squad.
"I am very happy and have been so since the first minute," he continued. "It was an opportunity that I wanted to take.
"Now I like English football a lot because of the style of play and the speed of it."
Manchester City are set for another season in the Champions League and Navas believes that the club can progress after two disappointing campaiagns.
"City is a team that should aspire to win everything," he added. "We are strong and have to have confidence and we should remain calm.
"We have started well in the league although our main aim is the Champions League because we want to go a long way in the competition."