Martinez is definitely a talent! He was bad against Barcelona but the entire Bilbao team was, Bielsa messed up his tactics so they looked like 11 headless chickens who didn't know what to do.
I think Martinez could be a better midfielder than defender here though, and it'd make sense at least initially as we could then keep Kompany/Lescott together and we could perhaps try Martinez occasionally at the back. Martinez as a defender is very good but he's quite reckless and has given away a fair few penalties, and you know teams with divers like the scum and dippers would exploit that.