Bayer Leverkusen goalkeeper Fabian Giefer was kept in hospital after being diagnosed with concussion following an 84th minute clash with Mainz 05 striker Eric-Maxim Choupo-Moting on Sunday. Giefer stayed on the field for the remainder of the match, which Leverkusen lost 2-0, but after a post-game scan was told he must remain in hospital for observation for at least two days. Club officials said he is likely to be sidelined for Sunday's home match against Werder Bremen, giving Leverkusen coach Robin Dutt a potential dilemma. German international Rene Adler is out until September after undergoing knee surgery while David Yelldell suffered a heavy knock in the German Cup defeat to Dresden. If Leverkusen fail in their attempt to sign young Stuttgart 'keeper Bernd Leno, last season's Bundesliga runners-up -- who are winless after two games this campaign -- have no choice but to call on Yelldell.