Sagna admits that Mangala is a little rough around the edges but claims that his fellow Frenchman has the potential to reach an elite status in the game.
"He has everything he needs to be a top player and I'm sure he will be. To me he will be one of the best defenders in the world, but he's still young and there's a lot of room to improve," the right-back told ESPN.
"I'm everything to him. He's like my twin brother, my son. I see him more than I see my family, so our relationship is really important.
"It's my responsibility to help him. I remember when I came here I didn't speak proper English. But he's a lovely guy and he's settled in well."
Mangala has made four appearances for the Citizens so far this season.