WordNet sense 1 (an athlete who plays only when a starter on the team is replaced):

reserve, second-stringer, reserve

WordNet sense 2 (someone who takes the place of another (as when things get dangerous or difficult)):

backup, fill-in, relief, reliever, stand-in, backup man

WordNet sense 3 (a person or thing that takes or can take the place of another):

replacement

WordNet sense 4 (act as a substitute):

deputize, deputise

WordNet sense 5 (be a substitute):

sub, fill in

WordNet sense 6 (put in the place of another; switch seemingly equivalent items):

replace

WordNet sense 7 (capable of substituting in any of several positions on a team):

utility

WordNet sense 8 (being a replacement or substitute for a regular member of a team):

second-string

WordNet sense 9 (being of second rank or importance or value; not direct or immediate):

secondary

WordNet sense 10 (artificial and inferior):

ersatz

WordNet sense 11 (contrived by art rather than nature):

artificial, unreal