Why does Candy feel his usefulness is gone?

Candy is limited in what he can do because of accident that rendered one of his hands useless. He's afraid that his boss will dismiss him because he isn't able to do his fare share of work.

"Candy sat on the edge of his bunk. He scratched the stump of his wrist nervously. "I got hurt four year ago," he said. "They’ll can me purty soon. Jus’ as soon as I can’t swamp out no bunk houses they’ll put me on the county."