Yes there are certain players who are very important in every team settings but it doesn't necessarily mean that the team has to struggle whenever they are not available for selection which is why every football club has multiple players available at all times.
Manchester city used to be that team that has a double squad, I mean the first eleven and those other exceptional players who are on the bench yet they have so much influence to the extent that they are capable of influencing the outcome of a match, but i guess that is no more the case at Manchester city, which is why I still think it is time for the club to start anticipating getting new players because the team seems to be having a lot of ageing players in squad.
The squad depth can help against unexpected events so that if an injury occurs to a core player it does not affect the outcome of the game. Pep Guardiola does not have a good squad depth so it will be a bit difficult to rotate when his core players get injured and that is evident in how inconsistent their performances are this season. This is different from the previous seasons where the reserve squad they had was no less good than the core players so that even if the players were injured it did not affect the results of the match.
New players are indeed needed because De Bruyne and several other players are not in good form and maybe age factors also affect. So with the presence of several additional players will make Pep Guardiola have options in doing rotation and most importantly not affect the team's performance in the competition.