Massaro, who is currently working as a pundit in his homeland, says that the Argentine hitman is being lined up as a replacement for Karim Benzema.
The Inter captain has long since been linked with a move away from the club and those rumours have resurfaced following a prolific start to the new season that has seen him net 13 goals in 13 Serie A games.
However, despite his goals, Icardi has not always been a popular man at the San Siro, angering Inter fans after threatening to bring 200 criminals from his native Argentina to have a fight with a section of the ultras that had been criticising him.
Speaking to Spanish publication 'AS', Fassone claimed that a deal to take the player to the Bernabeu next summer is already done, saying: 'I can tell you today that Mauro Icardi will be a Real Madrid striker next year.
'His arrival will be at the expense of (Karim) Benzema.'
Given that the 24-year-old is under contract in Milan until 2021, he is likely to fetch a significant transfer fee.
Benzema meanwhile, has attracted plenty of criticism for his lack of goals so far this season, but he silenced some of his doubters by scoing a brace in the 6-0 thrashing of APOEL on Tuesday.