Have to agree with Zdarg and Greshik. With 1upt, setting a worker on auto is bound to cause major headaches and bottlenecks, especially with the less than stellar worker AI and pathfinding algorithms . . .
OP, you might just want to have like a ranged unit (archer,crossbowman) occupy a central location in your worker territory, he will be able to better protect your workers (especialy if he is on a hill for the sight bonus).