Witnesses told WSB-TV, Atlanta, the victim, a 35-year-old man, was upset about the Falcons' 28-24 loss to San Francisco Sunday night and began arguing with a 49ers fan in the silver lot outside Gate D.
"Well, one of the Falcons' fans just kept talking, telling him, 'You ain't going to come here and win and nothing,' and he hit the man, and that's what happened. The man went into his pocket and cut him," witness Tony Morgan said.
Police on the scene used pepper spray after the incident became chaotic, officials said.
The victim was transported to a hospital where he was reported in stable condition.
The attacker fled the scene and remained on the loose, but officers said they believe they had recovered the knife used in the fight.