Police have confirmed 41-year-old Babak Rafati had tried to commit suicide while the local press reported the referee of Iranian origin been found in his hotel room having cut open his veins.
According to news agency SID, his condition has been described as critical.
Bundesliga coaches have already expressed their shock at the news, just over two years after Germany goalkeeper Robert Enke took his own life after battling with depression for several years.
"I know him very well, the news has shocked me greatly," Hannover 96 coach Mirko Slomka told German Sky after hearing about Rafati.
Schalke coach Huub Stevens was dumbfounded.
"It's hard to believe that something like this can happen," said the Dutchman. "I am lost for words. Health is always the most important thing and football is only a minor matter in comparison. I can imagine no one wants to play after hearing the news."
Rafati has been a Bundesliga referee for six years, ironically his first match having also been between Cologne and Mainz, and he has officiated 84 top-flight German league games.
He had been a FIFA referee since 2008, but was not nominated by the German Football Federation (DFB) to referee this year at international level.