I don't think you can swap out an entire side, or at least half of one, and expect a good showing. Understanding teammates matters. Chemistry matters. Yes, players can play to a certain level, and generally do so regardless of teammates. But when the chemistry and understanding is there, the players elevate their play above their base level. Milan do not have that elevation at the moment.
After the transfer window was complete, many football fans thought that AC Milan was finally fully revived. They made 10 middle class good signings, such as Biglia, who was Lazio's captain. Another one was Kalinic, who was Fiorentina's ST. They convinced Donnarumma to sign a new contract, and got Leonardo Bonucci to sign for them, which was the most unexpected of all and got many people thinking AC Milan would finally challenge. However, it did not go exactly to plan, the side still has too many problems.
Manager
Before everything begins, I think everyone knows that in order for a team to really begin to challenge, regardless of the players the teams consists of, the manager must be allowed to have full freedom. Good managers are those that have freedom to make bold decisions. For example, James Rodriguez from Real Madrid, was a guy who was expected to be the future of Real Madrid cost them 78 millions euros. As soon as Zidane came in, he created a system in which James Rodriguez was not involved in. He had all the freedom to make changes, in exchange he won Barcelona in his first El Clasico and won the Champions League in his first season
For Milan, Montella does not have that. Last season, he finished 6th, and won the club's first trophy in nearly 6 years, with a rather average squad. At the beginning of the season, he used his preferred 4 3 3 formation, and to be honest it was quite convincing. However, he was practically forced into using a system in which most new signings can play in, and it's clear to see that simply has no balance. Against Roma, Milan played 9 outfield players out of the 10 new signings. A.Donnarumma on the bench, and Conti was injured. The power of the owner is becoming too much, and it's really destroying football. Unfortunately, AC Milan's revival plans are being stopped by their owners, and worst of all their city rivals are getting better and better.
Bonucci
Milan literally had to do so much to get this guy. Firstly, they took Kessie's shirt number, and changed it from 19 to 79, and that was because Bonucci wanted it. Donnarumma was promised to be captain, despite his young age, instead they gave it to Bonucci. The reception he got was extremely welcoming, and the environment for him was really good. After many years with BBC, he wanted to try a new challenge, joining in a rebuilding period. However, playing with new players that don't have the experience of Barzagli and Cheillini is very difficult for him. He was the least experienced out of the BBC, and he went to assist Mussacio and Romanogli. He was probably the worst option of the BBC when it comes to pure defending.
Also, on paper, the Milan defense seems to be the one for the future, with Donnarumma, Romanogli, Mussacio and Conti all being led by Bonucci, but Milan cannot seem to find the rhythem of the team.
Money being spent
The money being spent personally wasn't as good as it could have been. I think it would have just been better to buy some mega players, like Aubameyang back from Dortmund. After all, he did spend many years at Milan and he knows the environment well enough. In fact, he once took an unauthorized holiday to Italy. Another option could have been Diego Costa, who was in a bad situation in Chelsea, he could have gone too. The players they bought were not bad, but if you want to see your new players in action, you don't buy 11 players and expect them to play with any chemistry, that simply won't work.
Anyway, what's your opinion on the matter, did AC Milan disappoint you?