Kaka scored his first goal of the season in Wednesday's 1-1 draw with Lazio, but failed to stop Milan's winless run stretching to three matches.
Allegri conceded that earning three points against Fiorentina tomorrow night is vital for his side's chances of recovering from a poor start to the season, which sees them sitting 18 points off the pace in 10th place.
He told reporters at a pre-match press conference: "From tomorrow we will have to start winning. We have to stay motivated because there are 28 games to go and our goal is to get one point more than last year.
"Kaka will play from the first minute tomorrow. He's doing well, he's growing. In his head he has the strength and desire to be a great champion again.
"Kaka is a great who can help his teammates grow. Off the pitch he is an example to all, he's educated, meticulous, serious and very tied to this club."
Milan have lost four of their opening 10 games in Serie A, including defeats to Napoli, Parma and Hellas Verona.