substitute player; substitution
Sports term for a player who replaces another during a game, or the act of substitution itself.
He entered the game as a substitute player.
The manager called a substitute from the bench.
控え選手 is a reserve or bench player, while 交代選手 specifically refers to a player who is actively substituting in or has been substituted.
補欠 is a more general term for a substitute or alternate, not limited to sports.
Compound of 交代 (こうたい, 'alternation, substitution') and 選手 (せんしゅ, 'player, athlete').