Yes the exact cause of the injury needs to be sorted out. Injury to the sciatic and gluteal nerves are not uncommon following hip surgery. However in the case of sciatic nerve injury you would expect to have developed the symptoms straight after surgery. Another possbility is the patient had a localised stroke or some other assault to the CNS as a complication of surgery.

Whatever the reason it needs to be properly investigated. This is a devastating result for a patient and further sinister causes of the problem also needs sorting out. The patient should have been referred to a neurologist with a full workup.