Adam Pearson revealed on Saturday that a fee had been agreed for the former Italy Under-21 international.
Owner Massimo Chellino also confirmed that Forestieri will complete his move to Elland Road later this week., although head coach Uwe Rosler admitted he knew nothing of a deal for the 25-year-old striker.
The Italian, who was born in Argentina, made 26 appearances for Watford in all competitions last season, scoring five goals - including two against the Whites in a 4-1 win.
But, until yesterday, he had not featured in any of the Hornets' squads this month since picking up a hip injury in pre-season after being involved in the first two friendlies played under head coach Quique Sanchez Flores.