regular player; first-string player
Used in sports contexts for a player who is a regular member of the starting lineup or first team, as opposed to a substitute or reserve.
He became a regular player on the soccer team starting this year.
This is my first time playing in a match as a first-string player.
控え選手 means a substitute or reserve player, the opposite of 正選手.
Compound of 正 (せい, 'regular, official') and 選手 (せんしゅ, 'player, athlete').