Because the injury is a disc herniation: a small amount of the nucleus pulposus or the "shock absorber" of the disc got squeezed out. It is a VERY SMALL amount. You get rid of the "squirted out" material with a 1 inch incision under a microscope.
There is a 5-10 percent chance that returning to physical activity too early could herniate more material. That's obviously not good because you've reherniated the disc and have to remove more of the extruded disc.
Bear in mind that Dwight was fully recovered, pain free, and ready to work out probably a few weeks after surgery (it is only 1 inch incision after all). BUT returning to physical activity too early gets you that 5-10 percent chance. Keep in mind that means there is a 90 to 95% chance that if he came back after only a month or so he would be fine.