The Ospreys are carrying out an investigation into their centre Andrew Bishop after a man was punched in the face and needed eight stitches.
Cardiff magistrates heard that it was an 'unprovoked attack' after a Sunday night bender in the city centre.
The Wales centre admitted assault and was made to pay fines and compensation of £1,000. Bishop was "intoxicated" after a drinking session with his Ospreys teammates, when he attacked bystander Allan Hornsey after he had asked if the rugby player was "alright".
Prosecutor Katie Pidgeon told the court: "The defendant was sat on the ground in a doorway with his head down. He was throwing punches with his clenched fists with both hands."
Stuart Hutton, representing Bishop, said that his client "enormously regretted" the incident, which was completely out of character.
He added that Bishop was usually "mild, polite and regarded as a gentleman who breaks up arguments on the rugby pitch".
