The 35-year-old enjoyed a prolific two-year spell at the club between 2005 and 2007 which saw him score 47 Serie A goals in 67 appearances.
He left the club for Bayern Munich five years ago, and was winding down his career with Al Nasr in the United Arab Emirates, but made a shock return to Serie A this summer.
"I cannot wait to get back on the field and make the fans happy," he told the Corriere dello Sport.
"I am glad to be back, I had my most beautiful years here. I want to start where I left off. I think we can play some good football this year. It is a great project."
The 2006 World Cup winner went on to say that he is looking forward to playing alongside midfielder Borja Valero, and is hoping to make his second debut for Vincenzo Montella's side against Catania on Sunday.
"Borja Valero is really strong. I did not know him, but I've been impressed in the days that we've trained together," he said.
"I have trained well and feel fine. I have put myself at the disposal of the coach and only he can decide if I play.
"The Catania game will be very difficult because they are a strong group. They have a settled squad, whereas we are a new team."