It's difficult to be patient about getting another CB. However, Barca does not really need that CB right now. The team could open the season with Pique, Masch and Bartra, no problem. The team needs this new CB before this transfer window closes. Doing this deal on the final day would be fine, then the new player would likely to ready to play before the second CB injury/suspension.
What is important is to get someone who is good enough, in this transfer window. Remember when Barca had problems getting an Abidal replacement, and ended up pretending Song could play CB?
If the team wants to go right up to the end, no problem if the end result is at least OK. But no more fake or non-existent CBs, please.
No, the team does not really need to add a second new CB. And whether or not Xavi leaves, no Xavi replacement is coming. Xavi will never be replaced, no matter who takes his roster spot.