Your information sources are mistaken. Pars fractures are almost non-existant with a microdiscectomy. In order to create a pars fracture, you would have to thin down the pars with a burr or rongier. The pars is generally not near the laminotomy (bone removed for canal access).
The need for fusion is normally about 1-10% if there has been a prior herniation and that need is normally many years after the herniation if at all. That means you have between a 90-99% chance of NOT needing a fusion.
Success rate for surgery for a standard disc hernation causing nerve root compression is 95%. The other 5% have chronic radiculopathy (see website for details).
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.