The NBA made announcement on Monday, after they say the violation occurred during the Nets home game against the Knicks at Barclays Center.
Kyrie Irving, who was spotted in the stands during the game, has missed all the team's games at the Barclays this season due to his unvaccinated status.
Up until this week, proof of vaccination was required for fans to attend an event in the arena.
Irving's inability to play during home due to New York City's COVID guidelines has sparked controversy around the league, including within Nets.
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Kevin Durant called on New York City Mayor Eric Adams to figure out what the Brooklyn Nets feel is a confusing vaccine mandate that now allows Irving to attend but not play in their home games.
"It's ridiculous. I don't understand it at all," Durant said.
Though that mandate has been lifted, there is still one in place for those working at the arena. The Nets were already aggravated because that didn't apply to visitors, allowing an unvaccinated opponent to play when Irving couldn't. They seemed even more frustrated after Sunday's game, when Irving's arrival created a scene at a nationally televised game.
Durant kept referring to Adams, who took office in January, by his first name.
"But he'll figure it out soon. He better. But it just didn't make any sense," Durant said. "Like, there's unvaxxed people in this building already. We got a guy who can come into the building - I guess, are they fearing our safety?
The mayor's office didn't immediately respond to a phone call and email seeking comment.
This is breaking news. This story will be updated as more information becomes available.
The Associated Press contributed to this report
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