We obviously need a CB, but tbh, now that I think about it, I think that more than anything, we need to bring back the pressing and the 6 second rule. Especially in big games and ESPECIALLY when we use shit defenses or are forced to use shit defenses due to injuries (like what's happening right now).
I mean, we've won CL ties with some pretty shitty defenses because of this. Remember Arsenal in 2011? Weren't Pique and Puyol out in the second leg? And I believe Busquets was one of our CBs in that game. Arsenal still didn't get a single shot, on or off target. We used makeshift defenses in the 2009 semis and final as well (Sylvinho-Pique-Yaya-Puyol in the final I believe). Though we were a bit lucky in the semi. Even in the 2011 semis and final, we used Masch at CB (and Puyol at LB for the semis).
We showed that we can still press really well and carry out the 6 second rule in the 2nd leg of the Milan game. If we press anything like that in the remaining CL games, I can still see us winning it, even with our shit defense.
Now, imagine a Barca side that actually has a GOOD defense pressing and using the 6 second rule. We'd be unstoppable.