Requirement for a successful bone graft? (photo)

I want to replace a bad tooth on my lower jar with an implant. But my dentist thinks I have lost too much bone on that tooth, a bone graft may not succeed. To succeed with bone graft the remaining bone on my jaw should be in a bowl or cylinder shape to hold the bone powder. I may not have a such shape bone socket after tooth extraction. The bone powder may be held by soft tissues and bone graft may fail. My dentist wants me to know the risk and decide whether I want to do this or not.