I searched when the post first came up, and found that ED is an issue that happens in a percentage of cases (saw figures between 5% and 15% mentioned). The cause doesn't seem to be known, as none of the nerves or blood vessels required for erections are near hernia sites, so several sites say it shouldn't happen, even though it clearly does. Sensory nerves to penis, scrotum and inner thighs are near hernia sites, as is blood supply to testicles, but nothing related to erections. I also couldn't find out if it gets better, and not knowing the cause, I can't even guess this. In the absence of any information, doing penile rehab as for post-prostatectomy would seem to be the best option.
Injections don't speed up healing, but it is important to have regular erections by some means or another to protect the tissues of the penis from permanent damage, and injections can play a part with this, as well as helping with the emotional side and enabling you to have sex too.