Well I did not know at the time that Herrera was out, so at this point in time I do not believe Carrick will play as a centre back. But SAF in 2009 and in 2012 already played with the idea of playing Carrick as a cb when in dire need (injuries). I do recall Carrick played a few games as a cb but I do not remember this very accurately. McNair did a fine job, but all young defenders make mistakes, they are just lacking the consistency and concentration. This will come with age, but these are one of the biggest reasons that there are practically zero young (20 and younger) good cbs.
Playing McNair now, in a total rebuild is something different then implementing talents in a well oiled machine. If McNair makes a mistake, a crucial one, versus Everton he'll get crucified by the press (along with LvG). I believe Carrick can offer the consistency, experience and build up play as a centre back, and that we really need atm. We need that experience to close out games, which we find very difficult atm especially with all the experienced players we have being injured or suspended.