By retiring, the 31-year old will now give himself some substantial rest in between international fixtures.
Koeman said: "If you reach the group stage of the Europa League and have international duty, then that is a really tough programme. His fitness is really good at the moment. It is up to the player to decide the best thing to do.
"I respect that and I would have respected it if he had made a different decision about his future as an international player.
"I think it is a good decision for him and a good decision for Everton. It means when the internationals come, he will have some time at home with his family."