Kaka rejoined Milan in a deadline-day transfer from Real Madrid last week, more than four years after his last game at the San Siro.
Allegri has revealed that the Brazilian has shown signs of nerves ahead of his Serie A return this weekend, but thinks that the playmaker still has plenty to offer his side this season.
He told reporters: "I think he will be in the starting XI. He is in good shape, he has trained really well and he has the enthusiasm of a young boy.
"I think he is a little nervous about his debut tomorrow and it is only normal that he is missing a bit of match sharpness. Kaka can give a lot to Milan.
"He has done some great things here at this club, he hasn't played a lot over the last three years in Madrid, but he has immediately put himself at my disposal. He spends a lot of the day at Milanello and he is a great example for the youngsters."
Milan have made a mixed start to their Serie A campaign, with an opening-day defeat to Hellas Verona followed by a 3-1 success over Cagliari.