Yes, RELIEVERS is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, L 1, I 1, E 1, V 4, E 1, R 1, S 1.

Is it a valid word?

RELIEVERS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

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
a pitcher who does not start the game

Also: stand-in, backup man, fill-in, relief pitcher.