I would go with "unskilled labor", "manual labor", or simply "worker." "Day laborer" is a possibility but there is an implication of short term work.
Many jobs have very exact names for specific helpers. The person who carries loads of bricks to a mason for example is a hod carrier. The film industry uses grips. Coroners have dieners. Dentists have dental assistants.
I disagree with "apprentice" since that means the person will, or aspires to, learn the craft. That is not necessarily the case with a manual laborer.