A story on the Santos website on Thursday night (UK time) spoke of a four-year deal and the player, born in Brazil but an Italy U20 international, is quoted as saying: "It is a unique opportunity and I will work hard to give my best here. I hope to play a lot of football."
Possebon, now 21, joined United in January 2008 from Brazilian side Internacional and made his first-team debut as a substitute on the opening day of the 2008/09 season.
Since then, however, he has managed just eight first-team appearances, although he's often impressed for the Reserves. He spent the first half of last season on loan at Sporting Braga in Portugal before returning to Old Trafford in January 2010.