The fact is other than Kompany, who we cannot rely on, Mangala is not worse than Otamendi or Stones. All are prone to errors. Otamendi's abysmal passing led to Bournemouth's chances on the weekend. He also has no pace at all and so has to dive in which is always 50/50 on him getting the ball.
Stones and Kompany are my preferred partnership but Mangala needs games. I do not k ow why we would look to sell him for half the price we paid and then spend another 30m on JFE. Mangala had a decent season at Valencia. Pep needs to play him, we will need a 4th CB without doubt this season.