Manchester city has been in consistent form in the champions league, they won all the teams in their group. Despite losing two matches in the premier league they are still in a favorable position and losing to Arsenal is not a prerequisite to see them as not being inform compared to last season and moreover, the leagues are still young so no teams performance can be judged a this early stage of the competition.
Manchester City must still be considered a serious opponent this season, both in the Champions League and in the Premier League because without even key players Manchester City is still able to provide very troublesome resistance to its opponents. Everyone can see how Arsenal won the match against Manchester City in the Premier League, even Arsenal was not able to score more goals in that match so they were only able to win with a few goals. So there is no reason to say that Manchester City is weak because so far Manchester City is still able to create good games in every match.