The transfers of good players are just potential opportunities (we don't know if they will take place). While the real situation now is this: Napoli played excellently at the beginning of this season and is only now experiencing a failure. Barcelona has been playing poorly all season and Xavi hasn't changed anything. I see that the bookmakers quote Barcelona's odds higher than Napoli, but most likely because of the "name", and not because of the real balance of power:
UEFA Europa League 2021/22 - FC Barcelona reach Round of 16?
Yes 1,69
No 2,05
If I am not wrong, then players those will transfer in this January window are not allowed to play in UEFA Europa League and Champions League, so this is not going to have any impact on these matches. Surely Barcelona is big name in Soccer instead of Napoli but with current form Napoli can beat them in this league game they started very good and stay at top for very long time, but few weeks back just because of few injuries they have some issues and now trailing at second spot with have some chances of claiming again their top spot from Inter.
These are just trap odds I am feeling Barcelona has no chance of reaching into next stage of this Europa League as Napoli can beat them, and they can advance into next round.
If nothing has changed, then the player who started playing in some European Cup cannot move to another team and continue playing in the European Cup there. Therefore, in theory, Barcelona can find several strong players on the transfer market (who have not yet played in European competitions this season) and buy them to strengthen. But practically, as you understand, the probability of such an event is extremely small.
Yes, these are the usual distorted odds that have emerged from the opinion of the crowd (most look at names, not facts). Napoli will be the favorite in this pair.