Anti-Islam activist and former English Defence League (EDL) leader Tommy Robinson, 34, found himself at the centre of the Old Firm rivalry after sporting a Rangers jersey on Saturday 29 July.
A number of Celtic fans allegedly attacked Fletcher's off licence in Sunderland, where Robinson had announced that he was conducting a book signing. The Hoops were in the north east of England for a pre-season friendly with Sunderland.
Robinson, real name Stephen Christopher Yaxley-Lennon, videoed the smashed front-window of Fletcher's and showed a man in a Celtic top on the floor with blood around his head.
North East Ambulance Service later confirmed that the unidentified male had suffered head injuries and was in a "serious condition".
Helen McMillan, the assistant chief constable of Northumbria Police, said: "Fans come here to enjoy the football; many with young children and we here at Northumbria Police do all we can to ensure they enjoy the day whichever team they support.
"Unfortunately we did have some incidents of disorder in and around Sunderland between Sunderland and Celtic fans and while police dealt with them quickly, it is a shame these types of incidents happen at all."
Robinson, a contributor to the alt-right Rebel Media YouTube channel, also criticised Celtic fans for showing solidarity with Palestine.
The Scottish club was hit with a fine of more than £8,600 (€11,200) by European Football's governing body UEFA in 2016 when fans flew the flags during a game against Israel's Hapoel Beer Sheva.
"All these girls who have got their cleavage out and their Celtic tops – take your cleavage, take your Celtic top and walk through Gaza, and show me your solidarity because you won't have any," Robinson said. "You'll have the biggest shock of your life, so stop hating on me."