cog in the wheel

a cog in the machine/wheel (an unimporant person in or a small part of a large complex, enterprise, organisation) — мелкая сошка, пятая спица в колеснице; мелкая рыбешка; небольшая птица
~винтик
~пешка (контекст)

He should not view himself, or allow himself to be defined by others, as the embodiment of a role or a cog in a system, in sports or anywhere else. (Sports in the lives of children and adolescents)

also
[low man on the totem pole]
small fry