I don't really have a good explanation either for Newcastle's decline, but I would not exclude the Champions League from the evaluation of their performance. Dortmund should have been an opponent they can beat. AC Milan I think so, too. They did beat PSG in that amazing game at home, but overall their performance wasn't sufficient to make it to the next round. But I think the Champions League should be taken into account because it's the same there as it is in the Premier League for Newcastle at the moment. They can't consistently perform on their best level. They have some games that are shockingly good and then they have these blackouts like recently against really some of the weaker teams. This season will be disappointing for everyone in Newcastle.
Newcastle is one team I thought they were going to shock the world with the way they play,I thought the only team who should have been capable of winning Newcastle are a team in the English premier league,but the results that I saw shocked me becausethey were the least team in their group.They showed weakness throughout the group stage,but what really amases me was how they were able to beat PSG in the first leg of the tournament,they really showed they were a team that could win any team in the tournament,but all their playing style and ability dwindled with time.Now Newcastle is now a team any team can beat.I hope they survive the competition in the EPL,and try to be in Europa league next season.
Newcastle can certainly hope to return to its best performance this season, but in my opinion ending the season in the European zone will never be easy. Newcastle are not performing well this season and in my opinion, Newcastle will not qualify for Europe next season if they don't build their squad in this transfer market.
Newcastle's performance is indeed very different compared to last season. But this is not because they failed to maintain their performance, but rather the improving performance of other teams should also be considered as a reason why Newcastle is unable to compete at the top of the standings. Being in 9th position with 29 points collected in 20 matches is not an impressive statistic from Newcastle this season. Newcastle have suffered 9 defeats so far and the bad thing is that 4 of those 9 defeats occurred in their last 5 matches. The bad trend is still ongoing and Newcastle is still very difficult to get maximum points at the moment. SO the match against Manchester City will likely make Newcastle worse off.