Matija Nastasić is among the most promising young defenders in Europe.
He joined Fiorentina from Partizan (1,5 years ago) who didn't give him any first team appearances, so he just played at Second Division-level prior to his move. According to rumours, Partizan only played him in their second team because the agent didn't allow them to. This is probably because of debts they've had to him previously, so what they did, in order to keep their best player, Cléo who took them to Champions League was to sell Nastasić to an agency who then, sold him on to Fiorentina. It's very shady business.
Nastasić is a tremendous player though, so it's not someone who's not good enough for the highest level who's just been put there by his agent. This guy went from the Serbian Second Division to Fiorentina and Serie A-level in 2-3 months after arriving in Italy. There's a good composure on the ball, though he likes to play the short pass more than the long ball. There's also an elegance in his defending which is impressive. Physically, he's good enough in Serie A which says a lot, although he'd definitely bulk up more if joining a Premier League-club. This is someone who stands on his feet and defends, doesn't slide into tackles. Nastasić is good in the aerial game, on both ends. The interesting part is that he actually played as a DM for the Serbian U16's in a tournament in Austria where tons of Italian clubs watched him.
I personally hope he stays at Fiorentina for another year, it'd be the best thing for his development.
Having said that, Bayern are also interested according to Serbian sources.