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Chelsea have seen steady improvement since Maresca's arrival, and their success in winning the Club World Cup should be a good indicator that they can do the same in next season's Champions League.
You're right, as you said, Chelsea shouldn't be underestimated. Personally, I'm quite confident in Chelsea, and they deserve to be placed among the favorites to win next season's Champions League. If that happens, it wouldn't be a surprise, as Chelsea have a pretty strong squad right now.
You are right in saying that Chelsea cannot be regarded as the most popular one; with a younger team, which is yet to mature. However, the history tells that they are capable of shocking people and the win over PSG in the Club World Cup definitely provides them with confidence. Their greatest weakness however is their lack of consistency since they are at times unable to maintain their good performances. I believe given the coaching of Maresca and the talent that they possess, Chelsea might very well go deep into the tournament. They are not to be dismissed lightly, though, since as you pointed out they can create some massive upsets.