I'd like to add another point too... Mangala is currently in our 1st choice back 4. People saying he can improve with regular football in a loan deal... he's getting regular football here. If I was Mangala I'd be well fucked off and would start looking for a new club. It would be a huge slap in the face and an admission from City that they don't rate him.
And what is the guarantee that Otamendi won't struggle? The most expensive defender and now we're sending him on loan, as Martinez said the transfer window should end once the season is underway.