According to The Herald, the incident took place after an argument broke out between two spectators on the west pavilion on Saturday night.
Shortly before the final whistle, some pushing and shoving led to a punch being thrown, and when another person tried to intervene, he was stabbed in the upper back.
The newspaper reports that the assailant quickly disappeared into the crowd. One witness said the fight was over a refereeing decision.
Stadium security officials arrived shortly after the incident to assist the injured man. He was treated by medical personnel at the scene before being taken by ambulance to Greenacres Hospital.
Stadium management and the Eastern Province Rugby Union confirmed the incident, saying a 20-year-old man was stabbed shortly before 21:00.
The Lions won the game 41-22.