The Real Madrid veteran midfielder will join the Rossonero in a free transfer after the Club World Cup.
At his Milan presentation today, Allegri was asked about the Ballon d'Or winner.
He said, "We are waiting for Modric to arrive... There is the Club World Cup, he will arrive in August, he is an important and extraordinary player who needs no introduction.
"There is Modric, (Samuele) Ricci, then (Ruben) Loftus-Cheek who in my opinion is a very important player. Then there is (Warren) Bondo, (Youssouf) Fofana... We have excellent players. We will play with the three midfielders, then depending on the characteristics we will take to the field."
Allegri was also asked about Milan's interest in Juventus striker Dusan Vlahovic.
He added, "Dusan is a Juventus player. I can talk about the boy and say that he is an extraordinary person, I saw him young, when he arrived from Fiorentina."