Spinelli said: "It's practically a done deal.
"It was right to give the player the possibility to test himself in a different environment.
"After several years at Livorno, he deserves the chance to take a step forward in his career."
Diamanti joined Livorno in the summer of 2007 and is under contract with the Tuscan outfit for a further four years.
The player said yesterday that a transfer to Upton Park would "change my life and make me take a leap in quality".