I agree, Mertesacker may not be top tier but he is a very good defender and marshal of the back line.
I do overlook Ozil's start to last season because we really didn't give him much time after a grueling world cup and put him wide too.
Mertesacker didn't start most games for Germany but it's not like he had a break either. Not only that, he also had to partner a greenhorn like Chambers first and then for a long time he had Monreal as a partner with Chambers at right back.
It was but natural that our crisis along with a jaded Mertesacker was never going to do us any good.