It would make more sense to have signed a pacy wide player to play opposite Sterling and just let Silva and Aguero do whatever they want with acres of space and De Bruyne's best attribute seems to be his vision, decision-making and accurate striking of the ball (good passer on the ground, great crosser and pretty good shooter) on the counter attack and City don't do that a great deal...
but I don't think that will matter anywhere near as much as the fact that City now likely have 40-50 De Bruyne starts and he's a quality footballer and they lack depth. Bony and Nasri were the only proven attackers backing up 4 players right now and Aguero doesn't have a great injury record. Adding Navas to that group gives them a striker, AM and winger each for cover and whatever basic change Pellegrini wants to make.
Good news for Bony in the sense that De Bruyne can cross a ball, but bad for his odds to get in the first team, since Navas and Nasri were more likely to be beaten out and the 4-4-2 brought back at some point.
When everyone is healthy, Silva might be able to still run games from out wide if Sterling stays wide enough.I don't think this fixes their CL problems (though Otamendi has a chance to do so) but it will help them get through that insane group and definitely help them win the PL, for which they are looking favorites at the moment unless Chelsea sharpen or we find a great attacker and a (presumably internal) solution to the LCB/GK questions.
I could see this Zabaleta's replacement, though probably just in the first XI rather then the squad with Sagna leaving. A lot of space to cover and defensive decisions to make whether it's De Bruyne or Silva failing to cover the FB out wide. Or Sterling could play out right and Kolarov/Clichy will get exposed and replaced. LB is probably the next logical signing for City unless they can get a top CM or DM.