The problem is finding players who can be productive enough in terms of providing passes from the midfielder position in the transfer market is always not easy and even if there are players like that, they will not be the same as the quality that Rodri already has. So Manchester City also have to polish other players to be better so that some other players can also contribute in making classy passes to their front line players. Because if Manchester City cannot change this in the rest of this season, of course Manchester City will not be able to go far in the Champions League and also in the EPL this season.
I think there are players that Manchester City can sign who can help them at the moment until Rodri returns. Players like Bruno Guimarães can fill the Rodri shoes for a while but they can't buy a player like that because what they need is a short term and this is where the trouble is. In a January transfer, teams are not ready to ready to release their star players mid-season so Manchester City has to look for a player that's not in the top league of Europe who can play as a DM very well before Rodri returns.
Without a ball winning midfielder Manchester might not go very far in the champions league because if they play the way they care against top teams then they will get knocked out easily. Gundugan and Kovacic are not hardworking, they easily get tired and they don't know how to constantly press teams.