reserve player; substitute player
A player on a sports team who is available to replace a regular player during a game.
He is still a reserve player, but he might play in the next game.
補欠 is a more general term for a substitute or alternate, not limited to sports.
控え選手 is a near-synonym for a substitute player, often used interchangeably with 予備選手.
Compound of 予備 (reserve) and 選手 (player).