Has anyone seen a pitch map showing Mendy's average position playing full back against Bournemouth, compared to Sane's average position playing wing back against Everton? I haven't, but it wouldn't surprise me at all if Mendy was more advanced than Sane. It might have notionally have been 4 at the back on Saturday, but Mendy spent very little time in a defensive position. Obviously I don't know whether he just isnt fit enough yet to get up and down the pitch (and he should be fitter a fortnight later) or whether it was tactical.
If he is going to play such an advanced role against Liverpool then we would definitely need an extra centre back to cover behind him (assuming that Pep will not play two DMs). Otherwise Liverpool's front 3 will have acres of space.