The Germany midfielder is enjoying an outstanding season, with his form attracting the attention of Manchester United.
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Former Manchester City youth teamer Nmecha has a deal with Dortmund to 2028.
Kehl told Sky Deutschland: "Felix contributes a lot at the moment and is an incredibly important player.
"Don't worry, I always do my homework."
This season, Nmecha has scored five goals and made three assists in 34 matches for Dortmund.