To do a task that one is not ordinarily expected to do because there is no one else to do it.
To use in place of something else, with the same function.
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 X in place of Y.
To use Y in place of X; to replace X with Y.
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.