"For the Champions League final there are concerns about Hummels," Klopp told reporters.
Dortmund, European champions in 1997, take on rivals Bayern in the first all-German final of the competition in London's Wembley stadium on May 25.
They are already racing to get offensive midfielder Mario Gotze fit in time for the game with the player recovering from a muscle injury picked up in their semi-final second leg against Real Madrid.
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