The Premier League champions have said they are 'shocked and saddened' by the news, with the supporter reportedly assaulted by a Club Brugge fan, and are now working with police in Belgium.
Pep Guardiola's side sealed a big victory in Belgium, winning 5-1 with goals coming from Riyad Mahrez [2], Joao Cancelo, Kyle Walker and youngster Cole Palmer.
It was the perfect night on the pitch, but there was some worrying news being reported off it.
A 63-year-old fan, who is part of the 'Blue Moon Belgium' City supporters' group, was attacked at a service station in Drugen after the match.
The fan was rushed to hospital and put in a coma, according to Belgian paper Het Laatste Nieuws, with claims local police were able to arrest five suspects.
The victim's son, Jurgen - co-chair of the supporters' club - wrote on its Facebook page: "My father was the victim of a cowardly attack for his City scarf at a stopover in Drugen. We hope the perpetrators are found and punished.
"We have been everywhere in England and the rest of Europe, never aggression. We come to Belgium once and you will be left for dead.
"Keep on fighting daddy. City love, Jurgen."
In a separate interview with HLN, Jurgen explained: "Afterwards we stopped at the parking lot along the E40 in Drongen. While we were already driving on, my dad was waiting outside for his two friends. There, a Club [Brugge] supporter approached him and pulled his scarf from his neck.
"When my dad asked for his scarf back, he received a heavy blow to the head, after which the perpetrators fled and left my dad for dead."
Jurgen added that members of the Belgian supporters' group - which has been established for ten years - regularly travel to the UK to watch City matches, yet this is the first time they have encountered such violence from rivals fans - and it was on their own doorstep.
"Every week, our members cross the Channel to follow City's matches, both in Manchester and away," he said.
"We have also attended numerous competitions in Europe. We have never had to deal with violence. And then you are happy that we will also play against a Belgian team, and you will come across that.
"There were enough witnesses and there are also cameras," he added. "But I cannot confirm whether they were also arrested. I really hope they don't escape their punishment."
Man City reacted to the news on Wednesday morning and confirmed they are now working with police at home and abroad to help find those responsible.
"Everyone at Manchester City is shocked and saddened to hear reports of an attack on one of our supporters after the Champions League game in Bruges last night," read a club statement.
"We are currently working with our counterparts at Club Brugge as well as Belgian and Greater Manchester Police to establish more information.
"Our thoughts and best wishes go out to the family and friends of the Belgian based supporter, who remains in hospital."