How do I handle an injured player, blood, or a possible concussion?

Injured player (Rule 3-3-6)
An injured player must leave the game if you beckon the coach (or athletic trainer) onto the court, or if the coach or trainer comes on without being beckoned. The coach can keep the player in by using a timeout to assess the injury.

Blood (Rule 3-3-7)
A player with minor blood has 20 seconds to get it stopped and covered (and the uniform cleaned) without leaving the game. If it isn't handled in 20 seconds, the head coach must either call a timeout or substitute for the player.

Concussion (Rules 2-8-5 and 3-3-8)
You must immediately remove any player who shows signs, symptoms or behaviors consistent with a concussion. The player can't return until cleared by an appropriate health care professional. Your state association decides who counts as an appropriate health care professional. You don't diagnose: you remove the player and tell the coach.