That's true, I expect both team to reinforce during the January transfer window. Also Napoli just got a new manager and before the meeting with Barcelona am sure the team will be executing the new manager game plans very well. That game is going to be an interesting one, it going to be who wants it more that will progress.
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.