Can a moderate impact cause a stress fracture?

If you got hit on the front of the lower leg it is most likely a stress fracture as there is not a lot of padding between the tibia and the outside world. The best way to tell if it is a stress fracture or ligament is to do an xray or scan and see if the fracture is on the bone. However, a moderate impact CAN damage a bone and a hockey puck is going pretty fast and so if it slammed into the bone with enough force it could hit and result in a hairline fracture. I would suggest the same thing as your doctor and get an xray.