The wide man will spend the entire 2015/16 campaign with the Turkish club with the view to a permanent move at the end of the season.
Chelsea wrote on their official website: "The winger will spend the 2015/16 campaign on loan in Turkey and the agreement includes an option to purchase."
The German international spent time on loan with Fiorentina and Anderlecht last term and has been a bit-part figure at Stamford Bridge since signing for the club.
The 26-year-old joined Chelsea in 2012 under Andre Villas-Boas' reign, but has failed to feature prominently for any of the subsequent bosses.
Juan Cuadrado - Spotlight on
He has made a total of six Premier League appearances for the south-west London club and looks set for a permanent move to Turkey of things go to plan.
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