Is it necessary that lawyers receive a solid training in subjects that are related to their chosen field of concentration?
No one can be an expert in everything. Try and you'll likely end up mediocre at many things.

Like a general practicioner in medicine, lawyers need to know their field, have a working knowledge of the germane aspects of other fields, and to know when to call in an expert. It is my understanding that criminal lawyers routinely call in expert (subject) witnesses, both to inform the court and to provide a more objective opinion than the counsel of either side could. 

Understanding criminal law is no mean feat, especially if we use understanding in the sense of "have it in my mind and can readily apply the relevant statutes to this case".  

Tellingly, there is no prerequisite coursework for law school. If a layer had to master a body of knowledge outside the law to practice, then that other body of knowledge would be required for admission to law school or taught during law school. The exception which demonstrates this heuristic is patent law.