Yes, you must have a worker to harvest that SPACE. Keep in mind that the worker controls the whole AREA (all connected spaces of the same field type) but only ever harvests the space he physically stands on.
Dude, I think you're confusing Key Harvest with Key Market.
To answer the original question- workers have no influence on the harvest action. You simply choose a group of connected field tiles and harvest the ones that are productive, i.e. flip them over.