The former Croatia star feels the Scudetto holders should not have allowed the veteran to leave San Siro for a Serie A title rival
Boban played for the San Siro outfit between 1991-2002 and feels the Italy international still has a few good years left in him.
"It was a resounding mistake to lose Pirlo on a Bosman," he told Il Corriere della Sera.
"We have seen in these first few games of the season how he is playing for Juventus and how useful he still would have been for the Rossoneri.
“Once upon a time Milan signed the best players in the world, now, though, they look at their balance sheet and club politics," he continued.
The Scudetto holders were beaten 2-0 by Antonio Conte's men on Sunday and Boban believes Pirlo played an integral role in the victory.
“The Milan midfield was overwhelmed by Juventus. We are talking about a slow and ageing department of the team. When Milan come up against a team like Juve then they’ll find it hard to hold on," he concluded.
The Italian champions have started the campaign slowly, winning only one of their first five games.