Rossi, who was Serie A's top scorer before suffering a serious knee injury in January and missing four months, came on as a substitute and scored in Fiorentina's 4-3 defeat to Sassuolo on Tuesday - his second game back.
Fiorentina boss Montella, a former forward himself, showered praise on the 27-year-old Rossi after the game.
He told his club's official website: "Rossi is well. He doesn't have fear and he is healed. His fitness condition will grow.
"He is a lethal player in the area even if he can also play away from goal. I am very happy for him. It was a pity that his goal wasn't enough to give us a draw."
The goal was Rossi's 15th in 19 league appearances and has sparked talk that he could be named in Italy squad for Brazil 2014.
"Rossi's goal was the only positive note of the game for us," Fiorentina sporting director Daniele Prade said. "Our only regret is not to have had Rossi fit for the entire campaign."
Fiorentina can guarantee a fourth-place finish with victory at Tuscan neighbours Livorno on Sunday.