Montella was sacked by Milan this week.
Macia worked with the Italian at Fiorentina and told TMW: "I'm sorry for his dismissal, we have worked very well with Fiorentina, he is a coach who makes a football style with innovative ideas. Unfortunately, Milan was expecting too much and in football the weak link is always the coach.
"It is difficult to build a team with 11-12 new players, and I read that when he arrived he did a similar revolution as Fiorentina.
"Milan however has a strong squad but needs time to gel."