Toni has arrived on a free loan from Bayern Munich for the remainder of the season.
"We are very happy to be here presenting Toni, an arrival who slots into a strong project," said Rosella.
"His arrival after the contract renewals for [Francesco] Totti, [Simone] Perrotta and [David] Pizarro shows we have a complete squad.
"Toni is one of the most important, if not the most important, signings of my era at the helm of Roma."
Director of sport Daniele Pradè was also pleased with the six-month loan capture of the man who won the Golden Shoe as Europe's top scorer in 2006 with 31 goals for Fiorentina.
"We have been talking to Toni for a month and got what we wanted. The lad too always desired this move to Roma and is very motivated.
"We'll see how he settles into the squad."