Well, they did not includes this part of story in manga so they might change it. It’s not unheard of that parts of story will be changed (or polished) from WN version.
It’s drastic but not the worst option considering what nanom can do, they can stop blood loss and block pain receptor. I doubt they can even reattached those parts within day (like our hospital can do).
You will be surprised with Japan weapon manufacturing capability (in real world). They are more than capable to produce advance weapon systems and they are still retain said capability in case they need them.