Its quality, not quantity.
Right now, honestly only Bartra is consistently a solid defender for us. That's 1 player. Pique is not consistent right now, Mascherano isn't a long term solution by any means and Mathieu is mediocre at best, its brutally honest but that's the truth.
You look at Bayern: Dante, Boateng, Badstuber, Javi Martinez as the emergency outlet, with true fullbacks in the form of Alaba and Lahm and honestly if push came to shove, Alaba could play as a CB and look very comfortable imho.
You look at Dortmund: Hummels, Subotic, Gunter, Sokratis all solid, reliable CBs.
You look at Madrid: Ramos, Pepe, Varane.
Hell even look at Juventus: Chellini, Ogbonna, Barzagli, Bonucci, Caceres who has gotten much better at Juve than he was here.
I'd take any of those teams CB pairings any day of the week over Masch-Pique again or even worse Verm-Mathieu.
I'm being harsh on Pique because he honestly is WC when he wants to be but his form has been terribly inconsistent for the last 2.5 years now. So even counting Pique, that's 2 true quality and reliable CBs while other teams have 4+ and especially for the style we play, I just don't expect players like Vermaelen and Mathieu to fit the bill in what we need. For depth playing the occasional La Liga or CdR game, sure. But against top quality in Europe and even in Spain, I question whether Verm in particular has what it takes.
It reminds me a lot of Song. He will stick out like a sore thumb here and people six months from now will be complaining about how poor he is and why we didn't get a proper CB. I swear its like people on here don't remember past 3 months or so, like a long-term memory loss or something...