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They have one big problem they do not play well all the time. In their last five games they have not played well and have lost three times. This has made them fall in rankings. Luckily teams they are competing with Manchester City and Newcastle have also played some bad games. This means Chelsea can still hold onto fourth place for now and try to qualify for UCL. But Chelsea position is not safe and we do not know if they will stay in fourth place. Their next game against Tottenham will be very important and it is hard to predict because Tottenham has not played consistently well this season either.
Yes, it is undeniable that if assessed one by one in its performance, there are several weaknesses of Chelsea that must be improved. Previously, their attack line was quite good, especially with the combination of Cole Palmer and Jackson. However, in the last few matches, its performance has declined quite a bit, unlike at the beginning of the season.
Jackson is one player that have gone from bad to worse just like you rightly said. From the beginning of the season, he was very good and was scoring goals as expected but I don't know what happened and he became very bad as his job.
On the other hand, Chelsea's defense is still quite bad, because there is indeed a lack of quality players. Hemm, they often spend a lot of money to buy new players, but not top players that they can rely on
Chelsea have some quality players at the moment but they still need to bring in new additions to the club to make them string and formidable. If I were Chelsea coach, I will definitely bring some new players to compliment what Chelsea already have.