Even though Manchester City managed to advance to the round of 16 as group winners after sweeping all matches with a win, they are actually experiencing a decline in performance. The player injury factor is the most reasonable reason, but a club as big as Manchester City should not be too dependent on just one or two players because they still have quality players who are ready to play in the areas needed.
Manchester City's performance in winning the UCL last season was very different from what they showed this season. With their current performance, I doubt they will be able to defend the UCL title for the second time in a row.
I think Manchester city seems to prioritize the UCL more than they do with premier league so most definitely they are showing a better performance in the champions league which looks more like priority to them and that's why in the champions league they do better but seem to experience a decline in the premier league.
Never the less I still believe Manchester city will still do well in the premier league and definitely get back on their feet, I'm sure Manchester city even if they don't win the premier league but will be able to finish top four and in the champions league aswell they will be able to get pass the round of sixteen even if they eventually don't win the title of which I know they are very likely to because they are very much experienced and have the capacity although this season they have more challenge to face with Bayern Munich, Real Madrid and other tough teams.