You need a definitive diagnosis as to where the lesion is. Obviously if it is a root avulsion it will need a surgical repair but the outcome is often poor. All roots must have been affected initially with the residual Erb's palsy left. This is the more common lesion of the brachial injuries.
As the previous writer said you or the surgeon have two options. Wait and see commonly 6 months or investigative surgery now. If it is an axontemeses it will come back in 6 to 12 months depending on the rate of growth of the axon. If it is a neurotemesis it is unlikely to return and will need surgical intervention. An Wikipedia reference-linkMRI should tell you this.
Personnally I would like to know exactly what I am dealing with.
Alan Esnouf