Costacurta was reacting after the club's 3-2 home defeat to Atalanta on Sunday left their Champions League qualification hanging by a thread.
Costacurta, who made 458 appearances for Milan and won five Serie A titles and the European Cup three times as a player, spoke to Sky Sport and did not hold back in his assessment of a club he has watched deteriorate since the change of leadership.
"It is not just about today's first half — it is the product of the Udinese game, the Juventus game, the Sassuolo game. It's four out of six games played that way. Today the players made an effort, but you lose so much clarity and mental freshness. The squad arrived out of breath at the crucial moment of the season."
He went further when the subject of the club's structural decisions was raised.
"Even those who don't know him know it was a mistake to dismiss Maldini. It is in front of everyone's eyes that many wrong choices have been made. Paolo was good in those years at creating a certain environment — and I'm sure he did, because I know how much it matters to him to create a foundation for a dressing room that gives energy, positivity and courage."
Maldini was dismissed in June 2023 following AC Milan's Champions League semi-final exit, despite overseeing a Serie A title in 2022, their first in eleven years. The decision was made by the club's American owners RedBird Capital and drew widespread criticism from the Milan fanbase at the time.
Sunday's result leaves Milan third with one match remaining. Fiorentina in fourth are three points behind but have a game in hand, meaning a defeat on the final day could cost Milan their top-four finish. Head coach Massimiliano Allegri is fighting for Champions League qualification that seemed comfortable eight weeks ago.