Historical Rules · 2 February 2024
Rule 310.10
Official source text from the 2 February 2024 Rules release
If a battle that isn’t being attacked has no player designated as its protector, or its protector is a player who can’t be its protector based on its battle type, its controller chooses an appropriate player to be its protector. If no player can be chosen this way, the battle is put into its owner’s graveyard. This is a state-based action (see rule 704).