I think we've bought a couple of average defenders in Otamendi and Mangala. Otamendi does have his qualities. He covers well and is very strong in the tackle.But both are rash, reactionary defenders. Otamendi's ability on the ball has to the most overrated thing I have ever seen going by what people make of him on here. The ocassional decent pass sandwiched in between sliced clearances and complete thin air clearances isn't good enough. Otamendi is better than Mangala but as I said in the summer, he was a flavour of the month defender who was signed partially because of a good goal scoring season in Spain. He scored 6 league goals I think and be scored the winner (iirc) at the Bernabeu to end Real Madrid's ridiculous winning streak. It was these headlining moments that lead to him appearing on people's radars. He still isn't much different from the defender who was loaned back to South America after starting his European career fairly poorly. He's an alright defender, but not close to top class.
Mangala is physically brilliant but mentally and technically not great. He lacks the intelligence needed to be top class and I'm not sure he will ever learn it.
The difference between us with Kompany and us without him isn't a coincidence. He is streets ahead of anyone else at the club at centre back. As bad as the rest of the team are at protecting the back four, Kompany seems to be able to hold everything together at the back. All Otamendi(to a lesser extent), Mangala and Demichellis seem to do is add to the chaos.