John Friel punched Danny Swanson as he left a restaurant with two of his teammates after a night out in Glasgow in November 2010.
In the same incident, ex-Dundee Utd youth player Matthew Flynn headbutted the club's left back, Barry Douglas.
Flynn was given 190 hours of community service.
The pair had also admitted punching passer-by John Stuart, who stopped to help Mr Swanson as he lay motionless on the pavement.
They appeared for sentencing at Glasgow Sheriff Court.
Sheriff Alan MacKenzie told them: "This behaviour was fuelled by a large quantity of alcohol and, as one sees all too often, violence ensued.
"The offences to which you have tendered a plea of guilty are serious, particularly Mr Friel - your behaviour is totally unacceptable.
"By the force of a single blow you, John Friel, rendered one of your victims unconscious."Mobile photograph
The court heard that Mr Swanson, Mr Douglas and former team mate David Goodwillie had been in Glasgow nightclub Karbon on 3 November and had met up with Scott Allan and Matthew Flynn, before going to a Japanese restaurant on Queen Street.
Procurator fiscal depute Bernard Ablett said: "The accused John Friel, who was not known to the Dundee United players, was also inside the restaurant.
"At one point Daniel Swanson attempted to take a photograph of Barry Douglas with another customer with his mobile phone, and John Friel put his hand up in front of the phone."
After eating their food, the Dundee United players left the restaurant just ahead of Flynn and Friel.
The court heard that only 19 seconds after leaving Flynn head-butted Mr Douglas then pushed him onto a parked car.
Mr Ablett said: "Danny Swanson's attention was distracted by this incident, and this allowed Friel to approach Swanson unnoticed from the side, and Friel then punched Swanson on the side of his head.
"Swanson immediately fell to the pavement, motionless for just under five minutes."
Mr Stuart was attacked by both Friel and Flynn after he tried to offer assistance to Mr Swanson, who was lying on the pavement, the court heard.
The court was told Swanson was taken to Glasgow Royal Infirmary and suffered swelling and bruising.
Mr Stuart suffered a broken nose which had to be re-set and had scrapes and swelling to his face.
Bob McCormack, representing Friel, told the court that the picture they were trying to take of a customer was his client's friend, who suffers learning difficulties and was called a "mongo" by the footballers.
He said: "Mr Friel raised his hand to stop this episode going any further and the photograph from taking place."