The Manchester City supporter critically injured in Germany is in a medically-induced coma, Press Association Sport understands.
The unnamed 32-year-old man is fighting for his life following an alleged assault shortly before City's Champions League last-16 fixture at Schalke's Veltins Arena on Wednesday.
Gelsenkirchen police have said the City fan suffered a "massive trauma to his skull and brain" after being punched and landing badly on the ground in a skirmish with two Schalke supporters.
It is understood the man had been at the game with his parents and sister, and the family regularly travel to City matches together.
City manager Pep Guardiola offered best wishes on behalf of the club at a press conference to preview Sunday's Carabao Cup final against Chelsea.
Guardiola said: "We knew it the day after. Of course it was terrible news. A family – a mother, father, sister. Unfortunately it happened.
"Of course our thoughts and best wishes, from all of us, to them. It was really tough news.
"Football is always a game, spectacular to everybody who enjoys it, but still right now this kind of thing can happen, unfortunately.
"Hopefully the police in Germany can solve the problem and the most important thing is the guy can recover and get well as soon as possible."
A member of City staff has remained in Gelsenkirchen to support the family and will stay there as long as necessary.
Police in Gelsenkirchen announced on Thursday that an serious investigation had opened with the support of their homicide division. Along with their counterparts at Greater Manchester Police, they have also appealed for witnesses.
City won the match 3-2 after coming from behind with late goals from Leroy Sane and Raheem Sterling.
The sides are due to meet again at the Etihad Stadium on March 12.
