O'Shea has played at left back for us quite a bit, Fabio not in many especially none of this magnitude. As such I think SAF went with going what Fabio has played in the most recently for us, while sticking with Mr "Depenable" O'Shea on the left.
I think Rafael will be the defactor right back barring injury. We've seen the need to spell Evra and hopefully Fabio can get a few games out on the left to slowly start taking over from Evra.
Then again if he is converted to a utility player (Fabio) then he may not end up with one position. I hope that doesn't happen - a lot of talent to try and play jack-of-all, master of none role.