Regaining grip and pinch strength are part of the recovery process from an arm injury. Nerves control the muscles. If there has been a nerve injury, rehab takes longer. If more than one nerve was damaged, then return of grip strength may be delayed even more.
Researchers in the Netherlands compared groups of workers with arm injuries. They found that patients with both an ulnar nerve and a median nerve injury had a slower recovery. Ulnar nerve damage especially affects grip strength. It takes longer for this nerve to repair itself.
One other factor should be considered. There is greater loss of muscle strength if the blood vessels to the forearm, wrist, or hand were cut or damaged. Healing and recovery take longer if this is the case.