Do we have to carry on pretending that a perfect "Rodri back-up" even exists? There's literally no player in world football who would happily sit out all the games Rodri plays and then bring his absolute best on the rare occasions he'd get to start. You could point at literally any player in the squad (except Ederson) and say "We should have signed a proper back-up" if they were to ever get injured long-term.
We have Kovacic, Gundogan, Lewis, Stones, and even O'Reilly and Gvardiol, who can fill in here. Of the many things Pep has mastered over the years with City, it's doing a heck of a lot with a small squad when big injuries arise. Mendy, De Bruyne, Laporte, now Rodri. We've seen in the early weeks of 24/25 that we can win well without Rodri and the January window isn't far away. You imagine we'll be looking at the likes of Kimmich or Bruno G then.
Gutted for Rodri personally because this means Vinicius Jr. will probably take the Balon d'Or, and at 28 he's no spring chicken anymore which means he probably won't be the same when he comes back. But we'll be alright. We've already made history, at worst this season we'll be finishing in the top four and going for all the cups. And we still have a squad that's more capable of winning this league, even without Rodri - it won't be comfortable but it'll be fun.