Did he sign a contract which stipulates that he must be vaccinated against Covid?
If not in the contract, then, the Nets can probably be sued for breach of contract.
Perhaps the normal course of action here is for the player to sue the team for breach of contract, and then the team files a third party claim against NYC.
If I was the owner of the Nets I’d at least look seriously at the possibility of breaking my lease on the arena for this season and playing the team’s home games somewhere else.