The ex-San Siro favourite, who spent 14 years there, feels the Italy international should have been kept on the club's books rather than being allowed to leave freely
Pirlo left the champions at the end of last season following an illustrious 10-year career at San Siro and moved to Turin on a free deal.
But Albertini, a former Milan midfielder and current vice president of the Italian FA (FIGC) believes his old club could regret their decision not to keep him on board.
"Milan taught me that you can make mistakes when you buy players, not sell them. What a mistake, getting rid of Pirlo," Albertini told Tuttosport.
"We are talking about a unique and irreplaceable player in Pirlo."
Massimiliano Allegri's men signed Alberto Aquilani on loan from Liverpool with a view to making it a permanent deal as Pirlo's substitute.
However, the Italian feels what the former Inter ace brought to the champions has dried up, regardless of replacements.
"Mark Van Bommel, Alberto Aquilani and Massimo Ambrosini are all different players," he said.
The Serie A showdown kicks-off at Juventus Arena in Turin at 19:45 (CET).
Pirlo, 32, is in doubt for the game after picking up a knock to his knee in Saturday's training session, but he was not excluded from the squad.