In the Pep era we defended well mostly because of the pressing system. If the works perfectly it doesn't matter who the defence is. We beat Arsenal 4-1 without Pique and Puyol. We won the CL final with Yaya at cb and Sylvinho at lb.
Defending is more about the team shape and everyone kong how to work in the system than individual talents. Obviously we want a world class cb if available to replace Puyol, but if none are available then get a stop gap for the mean time, our defence will improve under Tata even if we get nobody because he is improving our defence tactically.
The games last year we didn't press well. That's why we conceded so many.
In the Pep era, our starting CB duo was Pique/Puyol, both of whom were world-class then, unlike now. The pressing helped greatly, especially when they were missing, but we had a great CB duo. In Pep's last year, the pressing was still there (it wasn't as good as it was in his first 3 years, but it wasn't completely absent unlike last season), but it became clear that our current CBs aren't good enough, with Pique's drop in form, Puyol's aging, and Masch not being a real CB. Perfect example: Chelsea. We pressed well but still lost due to an error from Masch.
Its more about the fact that if we get exposed, our current defence is worrying. Sure, pressing is the best thing especially for a defence like ours but it doesn't guarantee that our opponets won't create chances. And there is where the problem exists, even one moment can do the damage if you don't have a secure defence.
This. Pressing helps greatly, and IMO, bringing it back was even more important than getting a new CB. It will ease the pressure on the defense, but the fact remains that our CBs aren't good enough.