reliever vs backup man

reliever and backup man both are nouns.

Word	Noun	Adjective	Verb	Adverb
reliever	Yes	No	No	No
backup man	Yes	No	No	No

As nouns, reliever and backup man are synonyms defined as:

reliever and backup man: someone who takes the place of another (as when things get dangerous or difficult)

Other synonyms of reliever include backup, fill-in, relief, stand-in, substitute.

reliever (noun)	backup man (noun)
a pitcher who does not start the game	someone who takes the place of another (as when things get dangerous or difficult)
someone who takes the place of another (as when things get dangerous or difficult)
a person who reduces the intensity (e.g., of fears) and calms and pacifies