The defender, a target for Juventus, has arrived to sort out his passport ahead of moving to Europe.
Mattioni flew out from Brazil without informing Gremio of his plans and faces a fine upon his return.
"Mattioni is a first class player, absolutely ready for the big leagues of European football. If I were to say, for me he is one for Milan. Yes, I would send him to Milan," said Raiola.