The below mentioned treatment methods are from a hospital in the UK. In the near future I will be communicating with doctors based in Sri Lanka to get there input on how treatments are done presently in Sri Lanka. Site will be updated once this information has been gathered.

Courtesy of The Leeds Teaching Hospital

What treatment will I receive after the assessment?
A member of the team will discuss the results of all of the investigations with you and whether an operation is necessary. If the injury to the brachial plexus is mild, and recovery is expected without surgery, then you will be seen in an outpatient clinic to help you with any problems that you may encounter with your arm, and to review your progress.
If it is clear that rupture or avulsion of the brachial plexus has occurred, then surgical exploration and repair of these nerves may be offered. If you have other more serious injuries, then the brachial plexus surgery may have to be delayed until these injuries are stabilized.

How are nerves repaired?
In clean cut injuries of nerves, each raw end of the nerve may be repaired (with the aid of an operating microscope) by re-attaching it to the other cut surface of the nerve. This end-to-end repair is usually not possible in the case of brachial plexus injuries, partly because the injury produces weakness at the shoulder muscles, which in turn allows the shoulder to droop consequently pulling the nerve ends apart. However, in the commonest injuries, the nerves are torn or stretched and the damaged segment of nerve either side of the rupture must be removed and repaired using grafts of nerves from the legs.
When nerves have been repaired, the nerve fibers then have to grow through the repair and out into the arm to the muscle or area of skin that they supply. These nerves grow very slowly and maximum recovery after nerve repair is lengthy. Furthermore, no nerve repair achieves complete recovery because of the impossibility of each individual microscopic nerve fiber reaching precisely and accurately the area it is best suited to supply.

What happens if I need surgery?
If you are not already on the plastic surgery ward, you will be given a date to come into hospital. When you arrive, you will be seen by the nursing staff, medical staff and anaesthetist. They will assess your health and this will also give you an opportunity to ask further questions.
The doctors will explain the operation again and will ask you to sign a form to say that you are willing for the operation for proceed. They may also draw a mark on the arm which is to be operated on.
You will not be allowed to eat or drink from the evening of the day before your operation. This is to prevent you being sick when you go to sleep during a general anaesthetic.
On the morning of the operation, you will be given a gown to wear and you may have some medicine to make you feel drowsy before a nurse and porter take you to theatre. In the theatre, you will be met by the nursing staff and the anaesthetist.

The Operation
All surgery is performed under general anaesthetic. The surgeon will examine the nerves to establish the extent of the damage and whether repair is possible. If they find that the nerves have been ruptured (separated) then the gap between the nerves will be bridged by a nerve graft. These grafts may be taken from less important sensory nerves on the back of the legs (sural nerves) or from nerves in the injured arm. These nerve grafts will act as guides through which new nerve fibers can grow and cross the gap caused by the injury, to reach the muscles and sensory receptors of the shoulder, arm and hand.
If the nerves have been avulsed, it is impossible at present to re-attach the nerves to the spinal cord (although research into this area continues). To try and regain function within the arm, intact nerves nearby doing less valuable jobs can be transferred to parts of the brachial plexus. The transferred nerves no longer allow their original, less important, movements but the signals they carry can be used to make the more important muscles of the shoulder and arm work.
Commonly transferred nerves include the nerve that normally shrugs the shoulder (accessory nerve) and nerves from the chest that supply the muscles between the ribs (intercostal nerves) which are transferred to muscles involved in stabilizing the shoulder and moving the elbow.
When these nerves start to recover, you will need to work very hard at retraining these nerves to move your arm and initially you may have to do different movements to make your arm work.

How long will it take for nerves to recover?
New nerves have to grow from the neck, down the brachial plexus, past the injury site and through the nerves into the upper limb in order to reach the muscles and sensory receptors. Nerves grow at an average of 1mm per day but will take longer to grow through the nerve grafts and across the sites of repair.
As the nerve starts to recover, you will initially get a twitch of your muscle. At this stage, you will be informed of how to strengthen this muscle by the physiotherapists. Recovery of your nerves and the rehabilitation of your arm can take from several months to several years.