The 26-year-old midfielder joined Stoke for £18.3m in 2016 and has scored twice in 28 appearances for the club.
His last game for Stoke was on 8 March 2017 and he spent 2017-18 at French side Toulouse and last season at Rayo Vallecano in Spain’s La Liga.
Meanwhile, Austria defender Kevin Wimmer, 26, has joined Belgian side Royal Excel Mouscron on loan.
Wimmer, who has played 19 times for the Potters, will spend the entire season with the Belgian Pro League club.
Find all the latest football transfers on our dedicated page.