@Emobot7 Mustafi is a one-dimentional defender, while Stones is dealing with Kolarov (left back) playing at centre and is already one of the best on-ball defenders in the league being only 22 years old. So if someone tells you that Stones is not good, he's openly lying to your face, like I said we are most probably talking about the future captain of Man.City, if he is already better in alot of elements than most Centre Backs, in some years he will be even better in basic covering and defending like John Terry was or like Vidic was.
If you were Pep Guardiola, how much would you be ready to expense to get Bonnuci? The Italian international has been a target for the Spanish manager since last year and seeing how Stones have not been up to the task this season, he might be ready to spend more to get him, the question is, how much is he ready to spend?