On Sunday night, Portugal blew automatic World Cup qualification with defeat at home to Serbia. They now face a playoff in March to reach the finals.
Santos revealed: "He asked to leave. He was the player who was losing his balance, for something it had to be.
"He entered the second half and said that his legs were too used, he couldn't do it anymore.
"Obviously it wasn't him who was going to be replaced. But it had to be."
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