replacement vs backup man

replacement and backup man both are nouns.

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

As nouns, backup man is a hypernym of replacement; that is, backup man is a word with a broader meaning than replacement:

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

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

replacement (noun)	backup man (noun)
the act of furnishing an equivalent person or thing in the place of another	someone who takes the place of another (as when things get dangerous or difficult)
a person or thing that takes or can take the place of another
an event in which one thing is substituted for another
a person who follows next in order
someone who takes the place of another person
filling again by supplying what has been used up