A football fan must do 100 hours unpaid work after being told he could have sparked big trouble when he fought a teenager on Trent Bridge.
Builder Mark Highton claimed in court that the incident was started by a 17-year-old Forest supporter after the visit of Newcastle United on December 2.
The pair were fighting and had to be split up by police as hundreds walked past. But Highton, 22, then threw another punch which hit the youth on the head.
Presiding magistrate David Herriot told Highton: "The large number of people increased the degree of harm caused by the offence. It was in a football environment and had the absolute potential for an escalation of the violence.
"Once one starts, it doesn't take much for others wanting to start. It's like a tinder box," he added at Nottingham Magistrates' Court.
Highton, who admitted using threatening behaviour, must pay £85 prosecution costs and the same amount as a government surcharge.
He was also banned for three years from attending Newcastle United or England matches home and away. He did not oppose this order.
Highton told a probation officer that he bent down to tie his shoelaces and was then struck by the youth. They began to fight.
Lauren Fisher, mitigating, said Highton had just become a father and had travelled from Newcastle to celebrate that event with his friends.
"He isn't a football hooligan. Football is not ordinarily his sport of choice. He is a rugby player and a rugby fan and not a football player," she said.
Miss Fisher described him as a "hard-working young man". He works six days a week as a builder for his local council, is buying a house and cares for his partner and two children.
And she said the other youth was "known to police as a Nottingham Forest supporter".
Leanne Townsend, prosecuting, said the incident took place after the match and was spotted by "officers tasked to monitor fans". Two young men were seen fighting and officers pulled them apart.
"They restrained the lad and as they were doing so, the defendant punched the head of the youth. When restrained, he continued trying to get at the male," said Miss Townsend.
Later police interviewed Highton, of Benwell Grange Terrace, Newcastle. He claimed to have been acting in self-defence and that both swapped punches before being parted.
Highton, who described his actions as "stupidity", had never been in trouble before.