Packer punched the man in Martin Place in November after being kicked out of a nearby hotel.
The 24-year-old appeared in Downing Centre Local Court this morning where he pleaded guilty to assault occasioning actual bodily harm and failing to quit a venue.
The court heard that Packer, who left the Warriors at the end of the most recent season and signed on with the Newcastle Knights recruit, had been asked to leave the hotel because he was so drunk.
He then walked 20 metres to a public seating area in Martin Place, where he got into a disagreement with another man.
Packer's lawyer, Murugan Thangaraj, said his client significantly overreacted and punched the man in the left side of his face.
The victim suffered two fractured bones.
Mr Thangaraj said he did not deny that the injuries were serious, but said the assault was not a king-hit.
Magistrate Greg Grogan said the 24-year-old's behaviour was cowardly and deplorable, and sentenced him to two years' jail.
The sentence came as a surprise to Packer's lawyer, who told Mr Grogan he had "absolutely no idea" he was considering jail time.
Packer appeared stunned as two corrective services officers led him out of the courtroom.
