substitute

act as a substitute

serving or used in place of another

be a substitute

“The young teacher had to substitute for the sick colleague”

put in the place of another; switch seemingly equivalent items

“substitute regular milk for fat-free milk”

capable of substituting in any of several positions on a team

someone who takes the place of another (as when things get dangerous or difficult)

an athlete who plays only when a starter on the team is replaced

artificial and inferior

“substitute coffee”

a person or thing that takes or can take the place of another