The funny thing is that the things being said about Otamendi now are more or less exactly what people said about Mangala last summer. People just like to moan and indulge in hysterics.
Last year our defence wasn't a big issue. Less than 1 goal a game in the league is fine. We were one goal off having the 2nd best defensive record in the league. City won the league with the exact same defensive record as us in 13/14 (37 goals conceded) and we had an even worse defensive record when we last won it in 12/13. Last season Chelsea conceded just 5 more goals.
Our defensive record will inevitably improve with Darmian, a fully fit Shaw and Schneiderlin and it's entirely possible that alone will close that 5 goal gap, so it's pretty clear we don't need another CB. If we want to compete in the CL then maybe we do, but that's something to think about next season. Our problem is our goalscoring.
Only two teams have won the PL by scoring less than 70 goals - United in 08/09, and Arsenal in 97/98, both with 68 goals. We managed just 62 last season and realistically need to find another 10 goals to have any real chance of competing. We should be competing with City for De Bruyne and Arsenal for Benzema, not City for Otamendi.