Since AC Milan was at home, I was expecting them to play nice football against Cremonese and beat them with at least 2-0 or 3-0, but I was surprised when I checked livescore and I found out AC Milan couldn't win or draw against the newly promoted Cremonese in their opening Serie A match this season. I didn't watch the match to identify AC Milan's weakness in the match but since you said AC Milan's defense was one of the reasons Cremonese defeated AC Milan, AC Milan still has little time to sign defenders before the transfer window closes if they want to challenge in the Serie A and Champions League.
Before the start of the previous season, we considered Milan to be well built and thought that the team would play for the title. But Milan's performance was erratic throughout the season. At the end of the season, the team was in eighth place in the table.
The same thing is happening again this season. Before the start of the season we saw that Milan's team is quite strong. Milan's players performed quite well in pre season matches. But they lost in the first match against cremonese. Cremonese is a team that has not been able to qualify for Serie a in the last two seasons. And this team will fight hard to qualify for Serie A. Despite being a very weak opponent, Milan lost this match.
What was Milan's major weakness in this match ? I would say their main weakness was their defense.. Cremonese's players only had 4 shots in the entire match and were able to score 2 goals. The performance of milan's attacking players will probably improve gradually. They missed a lot of chances in the match against cremonese. They need to learn from this mistake