The young Serb has been likened to United captain Nemanja Vidic and has been linked with a move to Old Trafford.
But Nastasic says: "My career is just beginning, I am not yet 19 years of age. I still have a long way to go. I hope in a few years to be able to say yes, I can be Vidic's heir."
On an offer from United, he added: "No. I am happy here, at Fiorentina. I want to stay here, my future is here."