
He will undergo further treatment at Bayer 04. A replacement has yet to be called up.
Germany U21 coach Stefan Kuntz said: "It's a purely precautionary measure. In discussion with Jona, our doctors and Bayer Leverkusen we have reached the decision to send the player home for treatment. Of course, it's a pity one of our leading players misses out on the start of the European Championship qualifiers but we do have options in central defence in our squad."

Black and Red talent development at the highest level – even off the pitch: Apprentices Jule Suermann, Leandra Bellia and Julia Niederheide have successfully completed their two- or three-year apprenticeships at the club. Bayer 04 honoured the trio at the BayArena on Tuesday.
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The Germany U19 team were beaten 2-0 by record champions Spain in the final of the U19 European Championship in Wales. Bayer 04 players Montrell Culbreath and Francis Onyeka (on loan at SV 07 Elversberg) were in the starting line-up and played the full 90 minutes.
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Bayer 04 Women have boosted their firepower with the signing of Poland international Paulina Tomasiak. The 24-year-old striker moves to Leverkusen from Gornik Leczna in her homeland on a two-year contract.
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Bayer 04 Leverkusen have signed France U17 international Kenan Doganay. The 1.90-metre-tall defensive midfielder, who has Turkish roots, joins Leverkusen from the U19s at French top-flight club Olympique Lyon. He will initially train with and play for the U19 team.
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