Berlin: Schalke 04`s assistant coach Sven Huebscher needed stitches after being hit on the head by a lighter thrown from the crowd during Saturday`s 2-1 home defeat to Cologne.
Robert di Matteo`s assistant was left dazed by the blow at Schalke`s Veltins Arena as he was helped from sidelines before the deep cut was stitched.
"We hope the perpetrator can be identified. That is unacceptable," said Schalke`s director of sport Horst Heldt.
Having scraped into the Champions League`s last 16 with a 1-0 win at Slovenia`s Maribor on Wednesday, Di Matteo`s Schalke had their three-match winning streak ended by Cologne.
Nigeria striker Anthony Ujah scored his fourth goal in five league games just after the break before midfielder Matthias Lehmann netted a penalty to put the visitors 2-0 up.
Teenage replacement Leroy Sane scored his first goal for Schalke by way of consolation five minutes from time on only the 18-year-old`s fourth league appearance as Schalke dropped to sixth in the German league table.