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The club acquired Gokhan Inler for 12 million euros.
The 27-year-old footballer was introduced at a press conference with a small show - he entered the room wearing "Napoli" shirt and a lion mask.
"I decided to come to this team a long time ago," the Swiss said. "This is a new stage in my career, for which I will dedicate all my strength. I want to celebrate many victories with the fans."
In the Italian championship last season, Inler played 35 games and scored three goals.
