Substitute

Austauschspieler

(player who is available to replace another)

Auswechselspieler

(player who is available to replace another)

Ersatz

(replacement or stand-in)

Ersatzspieler

(player who is available to replace another)

auswechseln

(sports: to remove from the field and bring on another player)

ersetzen

(to use in place of something else)