The 20-year old forward is on loan from Jeju United for all of 2014 and performed well for die Werkself in the German clubs midseason training camp over winter.
"Ryu's a quite different kind of player - we haven't got anyone like him in our squad. He made a good impression at the training camp as well," Turkey international Toprak told Bundesliga.com who likened Ryu to Manchester United's Japanese playmaker Shinji Kagawa.
"He's a small, really nifty playmaker. We sometimes jokingly call him 'little Kagawa' in training.
"That's the kind of player we've been short of. We'll see how he progresses."
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