Giefer was hurt in a collision with Mainz's Eric Maxim Choupo-Mouting in Sunday's 2-0 Bundesliga defeat but decided to play on.
"He will remain in hospital for one or two days for observation," last season's runners-up said on their website on Tuesday.
Starting keeper Rene Adler is out until September following knee surgery and substitute David Yelldell was replaced by Giefer for the Bundesliga start after his error-ridden performance in their German Cup first round exit to Dynamo Dresden in late July.
Leverkusen have qualified for this season's Champions League.
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