Hearts supporter Darren Manson was attacked by Motherwell hooligans as he walked to a train station with his mum after a game.
Hamilton Sheriff Court heard last week that Scott Notman joined in the "stramash" and punched the 22-year-old on the face.
The dad of three then bragged: "That's how it's done, boys, that's how it's done."
Notman, 46, admitted assaulting Darren, of Falkirk, to his severe injury, permanent disfigurement and impairment on August 16, 2019.
Window fabricator Darren underwent a four-hour operation to repair his jaw and also lost three teeth in the attack.
The assault came after Hearts had beaten 'Well 2-1 in a Friday night Betfred Cup tie at Fir Park.
Abby Seal, prosecuting, said Darren and his mum, Nicola, were walking towards Motherwell train station when they were surrounded by a group of 'Well fans in an underpass.
She stated: "They were abusive, but Darren, who was wearing a Hearts top, and his mum tried to ignore them. He was pushed against a wall by one male and his mum lashed out to try to push a second youth away.
"A third male then punched Darren twice on the face, causing him to stumble. After this he was punched again on the head and body.
"The accused Notman observed the assault, ran towards Darren from behind and punched him on the face. He shouted towards the group of males 'That's how it's done, boys, that's how it's done'."
Darren was in "a great deal of pain", but he and Nicola made it to the train station and then to hospital.
Ms Seal said he was transferred to a specialist unit in Glasgow for surgery. Almost two years on, Darren still has numbness in his lower lip and has a scar on his cheek from the operation.
The fiscal added: "His worst injuries have been mental in nature. He has had suicidal thoughts."
Notman was arrested after being caught on CCTV running into the underpass.
Sheriff Millar deferred sentence until next month, but warned Notman, of Airbles Street, Motherwell: "All options are open. It's a very serious matter."