I have a couple of comments.
First, T.J. Lang was not very good last year. I don't know if injuries held him back, but I was not impressed with him when he got in games. I believe he saw action against Chicago and Detroit.
The other thing to note is that 30 is not that old for a center. Scott Wells has a couple of years left in him and had his best season by far this year. In fact, Wells is probably a top five center in the NFC currently or close to it. Certainly, he is among the top half of 16 teams.
Finally, I recall McCarthy being optimistic about Brandon Underwood last year. Granted, Underwood had other things going on, which may have hindered his development, but I wouldn't read too much into his comments.
I personally believe the starting LG is not on the roster yet. Well see though.