Hamburg have been fined 35,000 euros after Werder Bremen captain Frank Baumann was struck by a mobile phone battery thrown from the crowd in the north German derby last month.
The German FA (DFB) opened an investigation after Baumann was struck on the left cheekbone as Bremen celebrated their equaliser in the 2-1 defeat.
A statement on Hamburg's official website confirmed the club had been "found lacking in the protection of an opponent".
The DFB have also warned Hamburg that another serious incident could lead to an even stiffer punishment.
The statement added: "In the case of a further serious occurrence the DFB sporting court has threatened Hamburg with a stadium ban or with having to play a match behind closed doors."
A statement on Hamburg's official website confirmed the club had been "found lacking in the protection of an opponent".
The DFB have also warned Hamburg that another serious incident could lead to an even stiffer punishment.
The statement added: "In the case of a further serious occurrence the DFB sporting court has threatened Hamburg with a stadium ban or with having to play a match behind closed doors."
Copyright (c) Press Association
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