You do not need a doctor to heal villagers. Any villager can be dragged to a sick villager, and after a few attempts, it will be succesful. I think the amount of skill that a villager has in healing, will mean that healing will be quicker and the success rate will be higher, however, weakened villagers cannot be cured, even by a master doctor. These villagers should improve if given less arduous tasks, and if you make sure that there is plenty of food in the food bin.

Savi