The 21-year-old Belgian had been expected to make a permanent move to Germany as part of a deal to sign Andre Schurrle, but the Blues allegedly don't want to part ways with him.
The young Belgian spent last season on loan in the Bundesliga with Werder Bremen and had been expected to return to Germany as part of a deal to take German international attacker Andre Schurrle to Stamford Bridge. 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," Reschke told German publication Bild. "It was not possible to connect it with the Andre Schurrle transfer."
Chelsea, meanwhile, is understood to still be hopeful of signing Schurrle, though sporting director Rudi Voller denies a transfer has been agreed upon by both clubs.