"This shows the consequences that can flow from a single punch," Judge Bruce Northwood said.
On September 13, Kerr had been travelling on the bus and drinking from a bottle of whisky.
The victim, who was sitting across the aisle, shared a small amount of liquor with the highly intoxicated man before things took an unexpected turn.
Near Rolleston, Kerr stood up to move to the back of the bus and when the other passenger attempted to have him return to his seat, he struck.
He punched the man once, connecting with his right eye and forehead.
The bus immediately stopped and Kerr was arrested.
The victim's injuries did not appear severe at first but subsequent scans revealed the "major trauma" the blow had caused.
He now sported a large scar across his face where surgeons had operated, the court heard.
A victim impact statement, outlined by the judge, described the ordeal that had followed.
After spending three nights in hospital, the man suffered the pain of nerve damage.
He said he now got tired easily and experienced frequent headaches.
While Kerr accepted he was at fault, defence counsel John Westgate said he continued to draw a blank when it came to his recall of the incident.
"This was a truly unusual offence in the sense that . . . the victim and the defendant were talking to each other, getting on quite well," he said.
"There's nothing to explain it."
Kerr had been experiencing some stresses in his life and had used alcohol as a crutch, but that was something he was addressing, Mr Westgate said.
Judge Northwood praised the defendant for attending Alcoholics Anonymous and accepted he was genuinely remorseful over the attack.
Kerr was sentenced to nine months' supervision and 40 hours' community work.
He was ordered to pay the victim $1475.
