I believe it is perfectly normal to experience numbness from any head wound. I was in a car accident, the glass from the windshield cut my head. I did not need stitches and the cut was minimal, which left very little scarring. You bet it hurt and left my head sore and numb for for few weeks. The tightness experienced from the swelling also went away in due time.
A similiar experience was felt post op. The donor area was tender ,a little swollen, and numb while healing. This appears to me as normal reaction and is part of the healing process with head wounds. I am not exactly sure how permanent nerve damage is caused, but I can tell you I have no numbness present. However, at around the 2 month post-op mark I started to worry the numbess wasnt going to go away, shortly after it started to subside.