Otamendi is disaster since his arrival. I remember when we kept clean sheet in first 6 matches with Kompany-Mangala in last season of Pellegrini. When Otamendi started to play chaos in defence started. He was terible against Everton, Leicester away and Monaco home games. He was bad even when we won in home game against Barcelona. Remember when Suarez humiliated him. Remember also how he could not handle Diego Costa at Etihad 1-3 against chelsea. Without any doubts Inigo Martinez is not in same class with Bonucci but in Pep's system he will be much more useful than Otamendi. Martinez is left-footed and comfortable with ball. Otamendi is more suitable with closed tense defences like Atletico Madrid or Chelsea.
I think that's probably a little unfair, you've picked out a few bad performances and broadstroked his whole city career with them. He's also had exceptional games for us too. I certainly don't think he's as consistent as he should be but he's a long way from terrible!