De Bruyne is a key player obviusly but i think Manchester City can sub him very well, in fact i really think this injury helped Manchester City thinking in the future of the team. Now they know they can play very good without him, so we can some news in the next summer transfers window.
Manchester City was not competitive due to the absence of key players, until they appointed Pep Guardiola in 2016/2017 EPL campaign, Pep is widely recognized as aggressive coach that has full ambition for the team to become great. He doesn't lack the full tendency but always curious about the performance of his team, ensuring for placing alternatives options in situations were the intended primary ones failed. Kelvin De Bruyne is injured and has not been with Manchester City for months now, probably might return in January because he's gradually making some vital recoveries, most football lovers can't wait for Kelvin De Bruyne to start games on the pitch for the Citizens.
decline in Man City performance is more dominant in the problem of players who are injured and unable to appear on the field with their team and De Bruyne absence has had a big impact on Man City performance because all of this has been arranged in such a way as possible by Guardiola so that the team becomes stronger but the weakness is in composition. If the player is damaged or there is player injury, it will certainly have big impact because as I said earlier, everything has been arranged competitively by Guardiola and I will not worry about this problem because Guardiola is very experienced and has also really studied various player characters and he is currently looking for a temporary player who has very high quality talent to replace De Bruyne and the reason for temporarily replacing De Bruyne is because Guardiola is still very enthusiastic about getting another UCL title.