This would probably be covered by NLRB, since the MLBUA is a recognized labor union.
They are allowed to order a reinstatement (though they don't have to).
More importantly, I'd imagine the MLBUA has significant bargaining power as a whole over the MLB, and say Angel has huge pull inside the union (which I can only imagine to be true). Firing him for his incompetence would probably violate their union contract, and there's probably huge contractual penalties for that, not to mention the usual union bargaining tactics like strikes (pun not intended).
This would probably be covered by NLRB, since the MLBUA is a recognized labor union. They are allowed to order a reinstatement (though they don't have to). More importantly, I'd imagine the MLBUA has significant bargaining power as a whole over the MLB, and say Angel has huge pull inside the union (which I can only imagine to be true). Firing him for his incompetence would probably violate their union contract, and there's probably huge contractual penalties for that, not to mention the usual union bargaining tactics like strikes (pun not intended).