Kevin Pezzoni was released from his Cologne contract on Friday after being threatened by fans.
Cologne, relegated from Bundesliga last season, saw no option but to allow Pezzoni to leave the club after the player had been subjected to online and offline threats.
"Things have happened that prevented Kevin from playing football in this club," Cologne coach Holger Stanislawski told the press after the team's 1-0 defeat against Energie Cottbus.
Cologne-based tabloid Express reported that five people had waited in front of Pezzoni's flat and threatened him last week. Reports also claim a note reading "Kevin, we will hunt you down" was put on the 23-year-old's windshield.
"A group of people have ambushed Pezzoni in front of his flat, accosted him and threatened him with violence," Stanislawski said. "He told me that every pass he played, he was afraid to make a bad pass. That is not tolerable. If a player is afraid to even cross the street then we have exceeded acceptable limits. We had to take this option."
Claus Horstmann, the director of the board, added: "Kevin told us he was no longer able to go into the stadium."
Pezzoni, who fled to Frankfurt, his hometown, wrote on his Facebook account: "I am happy to read with how well my decision has been understood. And how great a lack of understanding there is for mobbing, insults, violence and such things."
The Cologne players reacted with an open letter to their fans: "If we as players and as a team don't perform we take on criticism. But we won't accept that individual players are humiliated and personally threatened by individuals."
President Werner Spinner told the press: "The offenders will be handed stadium bans and will be excluded from the club, if they are members, and FC will closely work with prosecuting authority to prosecute them."
Reinhard Rauball, the president of the German league, told Welt Am Sonntag: "Escalation has entered a new phase. This is in no way acceptable. Everyone needs to realise now that from a certain point on there can be no tolerance. This is the case if there is violence in any guise."
Pezzoni had been a scapegoat for the fans for a long time and was attacked during the Cologne Carnival earlier this year. He was attacked and suffered a nasal bone fracture. After he had been brought on against Erzgebirge Aue last week, the media described him as "an own goal that was brought into the match."