The 21-year-old had been expected to make a permanent move to Germany as part of a deal to sign Andre Schurrle, but the Blues hope the Belgian will remain at Stamford Bridge
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But Michael Reschke, the Leverkusen general manager, has indicated Mourinho wants the 21-year-old in his squad at Stamford Bridge.
"At a meeting on Wednesday in Leipzig, Chelsea and Jose Mourinho made it clear that Kevin returns to Chelsea," said Reschke, according to Bild.
"It was not possible to connect it with the Andre Schurrle transfer."
Chelsea, meanwhile, are understood to still be hopeful of signing Germany international Schurrle, though sporting director Rudi Voller denies a transfer has been agreed.