Pirlo has decided to leave the newly-crowned Serie A champions after ten years at the San Siro with his contract due to expire at the end of the season.
Milan are already on the lookout for a suitable replacement for the Italian playmaker and are targeting a big name signing to please the fans.
"Milan have always played with top class players, we will replace Pirlo with a top class player," Allegri told the press on Saturday.
Pirlo, 32, is expected to join Juventus on a three-year contract but his agent has confirmed the deal is not yet done.
"He still has some details to sort out, but I think that Andrea Pirlo is heading for Juventus," said Tullio Tinti. "He'll make a decision in the next few days, but Juve is really the leading candidate."
Milan end their season on Sunday with a game against Udinese and the Rossoneri are expected to run the rule over Zebrette midfielder Kwadwo Asamoah, although Allegri would not confirm his club's interest. "Asamoah is a good player, but I'd rather not talk about him," he said.