It depends what the reason for Medical Discharge was. Normally they will try to rehabilitate your injury in Hunter Troop, however if the long-term prognosis indicates there will be a significant period before the known significance of the injury and how it will affect your continued suitability for service, you may be medically discharged.
Some conditions are a permanent bar to re-entry, others may need a surgical procedure to be carried out before becoming re-eligible, some also need a set period of recovery to elapse, some just need to be certified cured.
You remain in Hunter Troop until you reach the level of fitness through rehabilitation with the physiotherapists so that you can continue with Recruit Training.
There aren't many jobs that will effectively pay you full wages if you are unfit to work during your first 8 months. The Royal Marines do look after you extremely well.
If you cannot be effectively rehabilitated, you may be subject to a board to determine your future potential employability within the Corps if there is a realistic prospect of you recovering fully and completing training. You will hear many anecdotal tales of people that have been in Hunter Troop for months & others who have been medically discharged in very short order - the issue is the realistic prospect of the recruit completing training.