A player who is available to replace another if the need arises, and who may or may not actually do so.
A replacement or stand-in for something that achieves a similar result or purpose.
One who enlists for military service in the place of a conscript.
To remove (a player) from the field of play and bring on another in his place.
To serve as a replacement (for someone or something).
To use in place of something else, with the same function.
To use X in place of Y.
To use Y in place of X; to replace X with Y.