Normally, if a player has a minor injury, I would allow them to step off the field, receive treatment, then reenter with my permission, or be subbed at the next stoppage. If a goalkeeper is injured but wishes to remain in the game, does he have to give his jersey to another player while he is being treated and can he go back to keeping goal at the next stoppage?

USSF answer (September 10, 2011):
Unless a goalkeeper is so seriously injured that he or she must be removed from the game and taken for more complete medical attention and care, the ‘keeper will be treated on the field. If the ‘keeper needs to be treated off the field and might return after the treatment, then an outfield player must assume the uniform and duties of the goalkeeper. See the following excerpt from the Interpretation of the Laws of the Game and Guidelines for Referees in the back of your Law book, under Law 5:

Injured players
The referee must adhere to the following procedure when dealing with injured players:
• Play is allowed to continue until the ball is out of play if a player is, in the opinion of the referee, only slightly injured
• Play is stopped if, in the opinion of the referee, a player is seriously injured
• After questioning the injured player, the referee may authorize one, or at most two [medical staff persons], to enter the field of play to assess the injury and arrange the player’s safe and swift removal from the field of play
• Stretcher-bearers should only enter the field of play with a stretcher following a signal from the referee
• The referee must ensure an injured player is safely removed from the field of play
• A player is not allowed to receive treatment on the field of play
• Any player bleeding from a wound must leave the field of play. He may not return until the referee is satisfied that the bleeding has stopped. A player is not permitted to wear clothing with blood on it
• As soon as the referee has authorized the doctors to enter the field of play, the player must leave the field of play, either on a stretcher or on foot. If a player does not comply, he must be cautioned for unsporting behavior
• An injured player may only return to the field of play after the match has restarted
• When the ball is in play, an injured player must re-enter the field of play from the touch line. When the ball is out of play, the injured player may re-enter from any of the boundary lines
• Irrespective of whether the ball is in play or not, only the referee is authorized to allow an injured player to re-enter the field of play
• The referee may give permission for an injured player to return to the field of play if an assistant referee or the fourth official verifies that the player is ready
• If play has not otherwise been stopped for another reason, or if an injury suffered by a player is not the result of a breach of the Laws of the Game, the referee must restart play with a dropped ball from the position of the ball when play was stopped, unless play was stopped inside the goal area, in which case the referee drops the ball on the goal area line parallel to the goal line at the point nearest to where the ball was when play was stopped.
• The referee must allow for the full amount of time lost through injury to be played at the end of each period of play
• Once the referee has decided to issue a card to a player who is injured and has to leave the field of play for treatment, the referee must issue the card before the player leaves the field of play

Exceptions to this ruling are to be made only when:
• a goalkeeper is injured
• a goalkeeper and an outfield player have collided and need immediate attention
• players from the same team have collided and need immediate attention
• a severe injury has occurred, e.g. swallowed tongue, concussion, broken leg.