Arsene Wenger's center back conundrum

We've long waited for a squad that's both deep and healthy after many recent seasons sweating out every single match in the hopes that every player made it out OK and unscathed. The season's still in its infancy, but we're starting to see the effects of having health and depth at our disposal. In the past couple of matches Per Mertesacker has gotten the nod over Laurent Koscielny, who many could argue was our most valuable defensive player last season, to play alongside club captain Thomas Vermaelen at center back.

Of course, Koscielny's substitute role can easily be explained by the fact that he is still in the recovering stages from his injury, however Mertesacker hasn't done anything to suggest he's about to give up his spot in the starting XI. While it's always nice it is to have such a problem on your hands, it's still a problem nonetheless and Arsene Wenger surely must feel the pressure to successfully navigate the club and his squad selection through these next few matches.

So the question to you is: If Koscielny is back to 100% health and Mertesacker is still healthy and in top-form, who should be our second center back in the starting lineup at this moment?