I agree with you. Liverpool played well, but there was a slight decline in their defense. And we have to say, their defense isn't as solid as last season. There are several gaps that opponents seem to be able to exploit. Fortunately, this team's attack level is still top-notch, but Liverpool is still better.
Oh yes, Liverpool will also face a tough opponent in the Champions League. In the opening round, they will face Atletico Madrid in the Champions League. And I think that will be a good test for this team.
Liverpool in the last season shown that they were capable enough to go far in Champions League, but in a high competitive tournament like this, they need to have enough luckiness to go far. They have spent a lot of money for player purchases in this summer and are ready to defend Premier League title and also go farther in Champions League than a last season.
Yes, Manchester City should be looking for a replacement for De Bruyne. If they don't find one, they'll have to be more vigilant, as opposing teams will undoubtedly try their best to win. It seems many coaches now know where Manchester City's weaknesses lie, and Pep's movements seem to be becoming more predictable to other coaches, resulting in City's inconsistent wins.
Manchester City knew this and they even prepared for De Bruyne departure sooner than this summer transfer season. Knowing and preparing are good but it does not always lead to successful replacement on a legendary midfielder like De Bruyne. This explains why big clubs or national teams can not maintain their success a long time as they mostly can not replace old players by young players successfully.
Manchester City will need a long time to find another attacking midfielder at similar level of De Bruyne and in the meantime, their performances will be affected. Not only this, they have more problems, with other positions, other players and most important with their tactics.