It's not a Nashor's nerf. It's Fiendish Codex getting a different recipe that doesn't involve mana regen anymore. If Fiendish Codex is now cheaper to make in total (we only see the recipe cost there), then Nashor's Tooth has actually been buffed because its recipe cost was lowered. I can't think of anyone that builds Nashor's that wants mana regen so badly, anyway.
If the price is reduced, that's great then. Though I built this item for Kayle mainly, so the mana regen was actually rather useful. --- "Perhaps it's impossible to wear an identity without becoming what you pretend to be." - Orson Scott Card, Ender's Game