What do I do when a player is hurt, bleeding, or might have a concussion?

Injury (Rule 3-3-6)
If an official beckons you (or your athletic trainer) onto the court, or you come on without being beckoned, the injured player must leave the game. You can keep the player in by calling 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 without leaving the game. If it isn't handled in 20 seconds, you must call a timeout or substitute for the player.

Concussion (Rules 2-8-5 and 3-3-8)
Officials 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, as defined by your state association. CDC HEADS UP guidance for coaches is the same: when in doubt, sit them out, tell the parents, and don't allow a return the same day without a health care provider's clearance.