Rossi, who was fighting to return in time for the European Championships this summer, will now miss the tournament and be out of action for another six months.
"He's distraught," Castellacci told Radio Sportiva. "He expected to make a comeback in order to have a chance to make the Euros, but now I think he'll go back to the United States.
"This is a terrible blow for the athlete and the man, who I care about. He is a splendid lad and this setback is painful for all of us.
"He will have to undergo another operation and wait those six months to get back to football."
The Villarreal forward scored over 30 goals for the Spanish outfit last season.