A trainee accountant was part of a mob of Celtic fans who attacked a Sunderland convenience store before SAFC's friendly match against their team.
The Celtic fans walked from the city centre to Fletcher's convenience store in Tower Street, Hendon, after they heard former English Defence League leader Tommy Robinson was at the shop.
Robinson posted a picture on social media of himself wearing a Rangers football shirt, Sunderland Magistrates' Court heard.
"He was there as a guest of the owner of the shop," said Glenda Beck, prosecuting.
"As the bench may be aware, Rangers are bitter rivals of Celtic.
"Police became aware of the ongoing incident after footage was posted on YouTube.
"A flare was thrown at the shop. When police arrived at the shop at about 2.20pm they found Kasey MacKinnon lying on the ground, suffering from a head injury."
The court heard police examined YouTube footage.
"This showed Mr MacKinnon had wrapped his belt around his right hand, before rushing towards the shop and using the buckle to strike the front of it.
"He was taken to hospital for his injuries to be treated."
McKinnon, 18, of Bondyke Street, Glasgow, admitted threatening behaviour and criminal damage, both on July 29.
Patrick Andrews, defending, said: "Mr McKinnon does not fit the stereotype of a football hooligan.
"He is studying to become an accountant, and travelled down to the game with a group of older men who plied him with drink.
"The group saw the picture posted by Mr Robinson as they travelled down from Scotland, and decided to go to the shop.
"Mr McKinnon has already been punished in the sense that he was injured at the scene.
"Everyone else ran off, so he was the one who was arrested.
"He has seen the error of his ways, and won't be offending in this way again."
McKinnon was sentenced to a conditional discharge of 12 months, ordered to pay £105 costs, and £100 compensation to the owner of the shop.
McKinnon was made the subject of a football banning order preventing him from attending Celtic games in England and Wales for three years.