He is playing very well this season 5 games and 6 goals + an assist. Watford FC bought him for 2.50m but apparently, any team can get him if they pay around 20m. His current value is 20m according to transfermarket.
Watford struck gold in getting him when he was still unknown as he didn't debut with the first team yet.
Now though they will have to play defense if the rumors are true that any team that comes in with 20 Million can snatch him away. Fluminense itself is in a tough spot financial wise and if they get a 20 Million offer for him they will without a doubt sell him quick.