Can see walker there alongside otamendi and cancelo strating at rb.
Fern is ok there for certain games, when we expect to have possession and the threat of counter is small, but against the faster and bigger teams I'd prefer walker or maybe Harwood bellis.
But that said, pep has the time on the training ground to show how the position would work with Fernandinho there, so that is good enough for me if we do go with it.
Think Fern would be great against Liverpool. Him, as centre half behind Rodri, would be a solid defensive spine against the pace and attacking of Sala and Sane.