It's not only about not conceding goals, it's also about overall display of the player and whether he lets the opposite team create chances from his position. Ramos was weak against Sociedad and few other games which you could't say about Arbeloa. I'm majorly talking about defence, because a defender is supposed to defend, if he attacks well, fine, nothing wrong with that, but his major responsibility is to be solid at the back. Ramos wasn't solid at all, it is only due to Pepe and Carvalho great cooperation in the center which allowed his mistakes to go without consequence. I still recall him getting yellow cards, making silly, irresponsible tackles and at times even giving up marking (which he is probably best at when he plays in the center, that shows his weak form).
Tactics are new and he does have a new role which involves more contribution in defence, but it's not hard to notice that Ramos is more often better in offense than in defence while playing as a fullback. He still needs to up his game to be more efficient in his role as he was the one having trouble with his matchup wingers more than Marcelo did.
It's not like I'm looking for the weakest link in the team or someone who consistently causes trouble for our game, but Sergio was definitely the one who made most mistakes out of our current first team and I see nothing wrong, actually I'm glad to see Arbeloa play against Milan at RB, because you can hardly find a game where he was dodgy enough to be the weak link. He is more consistent than Ramos at RB and contributes more to defence than in offense + with current tactics which involve Marcelo having more of both offensive and defensive work, he would be the perfect kind of player to focus on defence more than on offense.