Actually I do not think that Manchester city are depending on one player as speculated, rather it's seems to me that the squad is in need of back up players because it obvious that when all these injury worries started at Manchester city there were no viable or suitable back up players in the the team and that was why the vacuum that Rodri left became so visible.
If the likes of kovacic were playing the way they Rodri was doing then no one would have noticed Rodri's absence, hence this goes to show how important it is for the coach to shop for more players in certain areas of the team, like the defensive area, the midfield and defensive midfield too, at least to help fortify the squad for the second half of the season.
One player does not determine the outcome of a match but one player has a great impact on the journey of the club. You yourself also said that Rodri’s absence was one of the problems that made Man City play inconsistently and that means certain players do have a significant impact on the team. De Bruyne plays a vital role in the midfield because his presence can make Man City play better and more coordinated.
But this season De Bruyne's performance has dropped drastically, making it difficult for the front line to score goals and even before that Haaland began to struggle to score goals because he did not have a measured pass given by the midfield or other colleagues. Some players are important and may have a big contribution to the club, although one player does not guarantee that the team can be much more consistent.