Tarka was the best defender in La Liga season before he joined us so he's got it in him. Mangala had "potential" when he joined us but I think we can safely say that he's not fulfilled that potential. Nor does he look even remotely close to doing so. The opposite in fact: he is a liability.
The man is 28 though and he dives in and makes as many daft decisions as Mangala does. I'm not defending Mangala as he is a lost cause also but you would of hoped for a man of 28 he wouldn't make anywhere near the mistakes he does.