Belfast Magistrates' Court heard bearded Cliftonville star Conor Devlin knocked Jim O'Hanlon unconscious with a single punch after the Glentoran winger repeatedly felt his facial hair.
Devlin, who was once on the books at Manchester United, was fined £400 and ordered to pay a further £400 in compensation.
The 22-year-old, who strenuously contested the single charge of assault occasioning actual bodily harm, is now set to appeal the guilty verdict.
The two footballers gave conflicting accounts of how violence flared during an encounter at a nightclub in the city's Odyssey complex in January.
Both had been out after their teams played in separate Irish Cup matches.
O'Hanlon recalled arriving with friends and then joining up with Devlin's group through mutual friends.
He claimed to have stroked the goalkeeper's beard once or twice in "friendly banter" and being told to stop it.
The Glentoran player said: "I didn't feel as if there was any aggression being shown or anything that would lead to what happened.
"I don't remember anything else from that night, the next thing I remember was waking up in hospital. I was covered in blood and had injuries to the eye and mouth."
O'Hanlon also said he required seven staples for a head wound.
But in cross-examination counsel for Devlin, of Hightown Rise in Newtownabbey, accused him of pulling or yanking his client's beard five times.
With CCTV footage of the incident played in court, defence barrister Martin Morgan contended that O'Hanlon finally grabbed hold with both hands and came in close as if to inflict a head-butt.
The lawyer said: "After asking you to stop a number of times he punched you once because he didn't know where you were going with it."
Following the blow Devlin put the stricken player into the recovery position and alerted door staff, the court heard.
Regarded as one of the most high-profile Irish League players, Devlin signed for Manchester United as a 16-year-old and made it to the Premier League giant's reserve team.
He has also represented Northern Ireland at under-21 level and twice been called into the senior squad for international matches.
Since signing for Cliftonville he helped the north Belfast club to last season's Irish League title.
Dressed in a light grey suit and dark shirt, he told the court he had taken only two drinks before O'Hanlon approached him.
According to his evidence the pair had never spoken before, and claimed the other player was being "boisterous and erratic".
Describing his beard as being longer and bushier at the time, the goalkeeper stated: "All of a sudden Mr O'Hanlon approached me and went straight for my facial hair, he forcibly grabbed my beard and tugged it."
He said it was sore and that he kept asking the Glentoran player to stop and go away.
"The last occasion was totally different, he seemed most aggressive and grabbed with two hands," Devlin claimed.
"The way he was approaching and pulling my beard I thought it would be easy for him to head-butt me.
"It all happened in a split-second. As he's standing, tensing, I thought I was going to be assaulted and I instinctively punched him once. He then fell to the floor."
Devlin, who told how he then used his first-aid knowledge to put O'Hanlon into the recovery position, told the court he believed he had done nothing wrong.
But prosecution lawyer put it to him that he had acted disproportionately and unreasonably, and could instead have simply pushed the alleged victim away.
After hearing all evidence Judge Nixon convicted Devlin of the charge.
As he imposed a fine and compensation the goalkeeper sat with his head in his hands.
Defence lawyers have said they intend to appeal the verdict.