Marshall suffered bad bruising to the back, head and body, as well as cuts.
Defence counsel Josh Lucas said Neilson drank alcohol only on "very rare occasions" and that night was one of those.
Neilson's actions were borne out of drunkenness, "bad peer influences and violent tendencies", the lawyer said. He had "out of misguided loyalty tried to help out his friend".
Neilson, who now worked fulltime at a minerals firm, accepted what he did was "disgraceful, idiotic", no matter who the victim was, Mr Lucas said.
Judge Tony Couch said Neilson had an extensive criminal history for serious driving offences, but none for violence.
He was especially concerned Neilson had jumped on Marshall as he lay on the ground, but the judge accepted Neilson had shown "a measure of remorse", and had expressed wishes to have a restorative justice meeting with the former sportsman.
Neilson's co-accused, Adam Kearns, 23, has pleaded not guilty and elected trial by jury in Invercargill. APNZ