Spot on HBHR, both are very different kinds of full backs and actually you could say that The Doss is more of a natural wing back than a full back, hardly surprising then to see how he's struggled, adapting to life in a new country with a new club, new team mates and new systems to adapt to and on top of all that having to grow accustomed to playing in a new position, i think a lot of posters can forget this when assessing his form to date.
I thought he was excellent whenever i saw him play for Udinese last year and was very excited when Rafa bought him but i was under no illusions that, like most players, it would take him a while to adjust, coming from a league as different as serie A is compared with the Premier League has most probably made that adjustment all the more difficult.