The reason for this is that a breast pump doesn't stimulate your nipples as well as a baby suckling, so they won't produce as much milk. You can make your self an over-producer of milk simply by nursing and pumping as often as you can. Before you return to work, let baby nurse unrestricted as often as possible. And, in between, pump your breasts. It may sound exhausting, but you will be building up the supply you will need.
Most hospitals rent breast pumps for a nominal fee, and they're of top-notch quality.
The longer you can stay home the better. The first six weeks are extremely important in establishing you milk supply, and will make it much easier for you to maintain the supply through pumping. Try introducing a bottle of pumped breast milk to your baby sometime between 3 and 6 weeks. This way, you can be sure that your baby won't refuse the bottle on your first day back to work. But only give the baby very occasionally, just to be sure that they will accept it.