If a player is injured and no substitute is available, who runs for the injured player?

When a player is injured and no substitute is available, the protocol established by baseball rules allows for the player who made the last out to serve as a runner for the injured player. This rule exists to ensure the game can continue without significant delay while also maintaining fairness by having a player who has already participated in the game take the place of the injured athlete.

Utilizing the last out as a runner is practical because that player is already warmed up and familiar with the current game situation, allowing for a smooth transition. It avoids the potential disruptions that could arise from calling in a player from the on-deck circle or any other location, ensuring that the integrity of the game is upheld while accommodating for injuries.

The other choices, while they might seem plausible, do not align with the established rules governing player substitutions and running for injured players, which is why they are not the correct answer.