I would advise you search for Dr. John P. Mulhall, from Memorial Sloan-Kettering Cancer Center. He has a very detailed book titled "Saving Your Sex Life, A Guide for Men with Prostate Cancer". It goes painfully deep into the details and mechanics of the different ED meds and injections.
There is no guarantee that the best of the drugs will recover ED issues if the nerves are removed or badly enough damaged, but doing nothing is right up there with doing nothing. In the worst cases, injectables have a very good track record, and can also help wake up an otherwise dead member nerve-wise.