substitute

A replacement or stand-in for something that achieves a similar result or purpose.

A player who is available to replace another if the need arises, and who may or may not actually do so.

One who enlists for military service in the place of a conscript.

To use in place of something else, with the same function.

"I had no shallots so I substituted onion."

(in the phrase "substitute X for Y") To use X in place of Y.

"I had to substitute new parts for the old ones."

(in the phrase "substitute X with/by Y") To use Y in place of X; to replace X with Y.

"I had to substitute old parts with the new ones."

A good that a consumer perceives as similar to another good, which decreases the demand for that other good