Goodwillie punched John Friel repeatedly near a Japanese restaurant in Glasgow city centre in November 2010.
Goodwillie lashed out after Danny Swanson, then a Dundee United club-mate, had been knocked unconscious by Mr Friel.
The Blackburn Rovers star had been due to stand trial yesterday at Glasgow Sheriff Court. However, the 23-year-old instead pleaded guilty to assault, and will learn his fate later this month.
Goodwillie had been on a night out with fellow Tannadice players at the city’s Karbon nightclub before they went on to the nearby Oko Express restaurant in the early hours of 3 November.
Mr Friel, 22, who was not known to the footballers, interfered as Mr Swanson took a photograph of a team-mate with a customer.
This sparked a confrontation outside and Mr Friel punched Mr Swanson, leaving him unconscious for five minutes.
Prosecutor Joe Stewart told how Goodwillie had come to his friend’s aid before learning that Mr Friel had been the culprit.
Mr Stewart added: “The accused made his way across Queen Street to approach Friel.
“He was partially restrained by a friend, but managed to break away, losing his shirt in the process.”
Goodwillie smacked Mr Friel before punching him five or six more times on the head and body.
The court was told that Goodwillie – who moved to Blackburn Rovers FC last August – also aimed a kick before Mr Friel staggered away, falling over a bin.
Mr Stewart said Goodwillie had appeared “emotional” before police turned up and detained the Scotland star and a number of others.
Mr Friel was given 240 hours of community service last August after he admitted the assault on Mr Swanson.
Goodwillie’s QC, Donald Findlay, yesterday said Mr Friel’s attack that night had been “cowardly and brutal”.
Mr Findlay told how Mr Swanson had a medical condition that had led his friends to fear his life could be in danger after being hit. The advocate said Goodwillie was “clearly very angry” at what had happened and confronted Mr Friel, who he said was standing “bold as brass”.
Mr Findlay added: “Mr Goodwillie accepts that this type of behaviour was not justified.
“It was largely driven by genuine concern for his friend who was in some distress.
“His [Swanson’s] life, for all that was known, could have been at risk.”
Goodwillie was fined in 2009 after he admitted attacking a rugby player following a row at a nightclub car park in Stirlingshire.
Sheriff Stuart Reid was yesterday about to impose a 100-hour community payback order for this latest incident. But he had to retract this when informed the offence pre-dated when such orders came into force.
The sheriff told Goodwillie the alternative to custody was instead community service – but he would require reports before sentencing.
It means Goodwillie will return to the dock later this month.
The footballer made no comment as he later left court.