Rosi made his Serie A debut on 28 May 2005, when Roma drew 0–0 with Chievo Verona. That was his only appearance in his first season with Roma. He then appeared in fifteen games during the 2005–06 season. He scored his first Serie A goal against Parma on 24 September 2006. Rosi made his Champions League debut on 18 October 2006 in a 1–0 away victory over Olympiacos. Rosi was also part of the A.S. Roma team that lost 7–1 against Manchester United in April 2007. His second goal was scored against Messina during the last home game of the 2006–07 season.