I don't think any of us will know how good he will be just yet. One of the biggest things he has going for him is his intelligence and ability to read the game - this is what impresses Pep the most I think. Having played at a decent level myself fullback was the position I could never get to grips with. I remember being brought on at right back for Peterborough and 20 mins later was taken off, I was completely lost (Shit actually & never recovered. I was released the following month). Eric had that look the other night so it maybe unfair to base any opinion on those 45mins - he's has been decent in all his other appearances and shows a lot of promise. I hope he stays and becomes our alternate CB, he has a lot of potential.
I do agree 100% about Otamendi and Stones - they are experienced internationals who went into panic mode and that nearly cost us the game. Ota has always redeemed himself by throwing his body on the line but often he has to do that following his own mistake. Stones has just lost confidence and it's hard to predict if he will be able to forge a long career with us.