Roma are unbeaten in Serie A this season, but a run of four consecutive draws has seen them slip to second in the table behind Juventus.
Tommasi said that if Roma manage to beat Fiorentina, another side who could challenge for the league this season, then it will be a big step.
"It 's an equally important challenge for the team to see if we can continue to aim for the title, or switch targets," he told Fiorentina.it. "We are at the beginning of the season but for both this is an important match.
"We must change the way we play [without Francesco Totti]. Some plays are not there and then the team must adapt because of this. They can make up for the absence of Totti extremely well and I think they can beat Fiorentina since they are are a team that let you play."
Tommasi played for Roma between 1996 and 2006.