I don't think you buy players to replace injured players.
The injury to Laporte exposes the poor quality of the cover. Last season, Kompany was the cover. Now it is an aging Otamendi or Fernandinho. I think City see Garcia and Harwood-Bellis as cover next season, and possibly first team players so I don't think we should buy a central defender. It's too short term.
I do think Guardiola has made the wrong call on Fernandinho. I don't think we should pay too much to comments at press conferences which may not mean too much but I am wary that he maybe on a mission to prove that he can convert Fernadinho to a central defender. Hopefully these comments ("I like Fernandinho playing in that position a lot," I prefer him there. That's why he plays") are nothing more than a manager refusing to criticise his player in public.
I hope that when Laporte recovers, Stones will partner him and this experiment ends.