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Barcelona may not be able to really reinforce their team by getting new players except they have probably gotten over their financial constraints but I think Napoli may be able to do that over the January transfer window this season because they may have the capacity financial as they have not publicly stated if they have any financial challenge recently.
If it were to be last season I doubt if Napoli will struggle to win the current Barcelona who has a weak form currently, but then both teams currently are unstable and inconsistent especially in their various local leagues, but in the UCL so far, they didn't do poorly in their group stage matches so I will not want to judge them completely based on their performance currently in their various domestic leagues as it seems in the UCL the give off a different performance hence it could be possible they will do better in the game than we have expected but it's not going to be an easy one for both teams.
Barcelona has more experience in UCL than Napoli and I see this as an opportunity for Barca. We cannot use the performance of a club in his domestic league to judge their strength in the champions league. We might be surprised at Barca performance in the knockout stage. On the other hand Napoli is somehow in the same situation with Barca based on their inconsistency this season and that has also brought down the performance of these two clubs. It will be a do or die affair but I think Barca will be the favorite in this match, because the coach will also determine the victory of his club and Xavi is a good coach no matter the difficulties Barca is passing through now. Who knows maybe Barca will give up in the Laliga race and focus more on the champions league.
I think there is no word surrender in Xavi dictionary, either in La Liga or at UCL. as you said, domestic competitions cannot be used as a full benchmark, because UCL is very different in many ways, including the atmosphere. The round of 16 will only start in february, which means that Xavi still has a lot of time to improve his club to be even better in the future. apart from that, Napoli will also do the same thing, their ambition to talk a lot at UCL is one of their current priorities.
It is clear that the knockout round is more difficult than the group phase, all contestants will try their best to beat each other from the first leg. no team can be guaranteed that they will advance to the quarter-finals, because we have seen so many surprises happen so far. Several other favorite teams have also experienced ups and downs in performance, therefore the room for surprises is wide open.
The transfer window must be used as an alternative solution to increase strength, but what needs to be paid attention to here is recruiting players who are mature and experienced. because when the competition is running at the current stage, the adaptation period should not be a new challenge, because the recruitment of new players is expected to have an instant impact on the club.