The N.B.A. fined the Nets $50,000 for allowing guard Kyrie Irving to enter the team’s locker room during Sunday’s game against the Knicks even though Irving had not been vaccinated against the coronavirus and thus was not allowed to be with the team at Barclays Center.

Irving had attended the game as a spectator, with a seat in the front row.

Under New York City law, Irving cannot play in games at Barclays Center because of a vaccine mandate for New York City-based workers who perform in-person work. While Mayor Eric Adams loosened some vaccine requirements this month, he has left in place the private sector mandate. Under the N.B.A.’s health and safety protocols, teams are obliged to follow local rules.

The Nets declined to comment.

During a public appearance on Sunday, Adams responded to a heckler who urged him to let Irving play: “Listen, you’re right. Kyrie can play tomorrow: Get vaccinated.”

Nets forward Kevin Durant called the rule “ridiculous” after the game against the Knicks on Sunday. He also criticized the mayor by name.

“It just feels like, at this point now, somebody is trying to make a statement or point to flex their authority,” Durant told reporters. “Everybody out here is looking for attention. That’s what I feel like the mayor wants right now: some attention.”

Minutes after the N.B.A. announced the Nets’ fine on Monday, Durant issued a statement through the Nets and softened his stance toward Adams.

“The last two years have been a difficult and painful time for New Yorkers, as well as a very confusing time with the changing landscape of the rules and mandates,” Durant’s statement read. “I do appreciate the task the mayor has in front of him with all the city has been through. My frustration with the situation doesn’t change the fact that I will always be committed to helping the communities and cities I live in and play in.”

Irving’s vaccination status has vexed the Nets all season. He has played in only 18 of the team’s 68 games, in part because the mandate has barred him from playing home games, and because he has refused to be vaccinated. Irving is allowed to play in road games where cities do not have vaccine mandates. Only Toronto, where the Raptors play, prohibits unvaccinated visiting players from competing.

Irving’s limited availability has contributed to the Nets’ free-fall from one of the best teams in the N.B.A. to one fighting just to make the playoffs with 14 games left. The Nets (35-33) currently sit eighth in the Eastern Conference standings, a game ahead of Atlanta (34-34); only the top eight teams qualify for the postseason. Unless Adams changes his mind, Irving will be eligible for only four of the team’s remaining games.

The downturn in positive tests nationwide and the lifting of other mandates had raised optimism within the Nets organization that Irving’s return as a full-time player was imminent. While Irving’s limitations under the mandate have received outsized attention because of his celebrity, the rule applies to New York City employees at more than 180,000 businesses, as well as other local sports teams like the Knicks.

Adam Silver, the N.B.A. commissioner, told ESPN last month that he felt the rule disallowing Irving from playing in home games “doesn’t quite make sense” because opposing players who are unvaccinated are allowed to play at New York City venues. Later that day, Adams agreed with Silver, saying that the rule was “unfair,” but also that lifting the mandate would “send mixed messages.”

The N.B.A. pushed for its own vaccine mandate for players before the season, but the players’ union refused to agree to one.

Irving’s attendance at Sunday’s nationally televised game against the Knicks created a spectacle. Los Angeles Lakers forward LeBron James, the N.B.A.’s biggest star, weighed in on Twitter during the game, writing that the law “literally makes ABSOLUTELY ZERO SENSE!!!”

He added: “They say if common sense was common then we’d all have it. Ain’t that the truth. #FreeKyrie.”