IMO Manchester City's problems this season are even more serious. Manchester City's overall performance last season was a bit erratic. Anyway, their defense cannot be considered too weak. Manchester City's defense is weak this season. Manchester city's last two matches were lost because of their weak defense.
Manchester City's forward's players' performance is weak. The new players have not yet adapted to the squad. The players need more time. The team has started badly. And city's opponents in the next three matches are even more difficult. City will play against Manchester united, napoli and arsenal respectively. In these three matches, City is likely to lose points.
Last season, Pep Guardiola blamed their bad performances on long absence of Rodri but the club provided him money to buy some players in winter transfer season. Their performances were not improved a lot since the transfer season and is it responsibility of Pep Guardiola as the head coach. I believe main reason is from Pep, not Manchester City players as in general they have very good player resource.
We can see the void of De bruyne in the City squad quite clearly. City have to extended the contract with de bruyne. It is a mystery to me why an experienced coach like Guardiola made the wrong decision to let De Bruyne go.
From Rodri to De Bruyne, it only borrowed reason but there are more problems than from absence of Rodri and departure of De Bruyne. This season they got Donnarumma as well as some players joined in the summer so Pep Guardiola's management quality will be fully revealed in this season. His tactics seems to be no longer operating well in Premier League with better adaption from opponents there so that they can constrain Manchester City better.