their defense is their strongest asset - last year, in the 2nd half of the season once Allegri figured out his formation and the injury crisis was over, Milan conceded only 7 goals, iirc.
vs Lazio Milan looked ridiculous for the first 30 min or so but that was due to their slow start and not being sharp yet due to lack of competitive games, not because their defense turned into crap over the course of 2-3 months.
I figure it will take Milan another 4-6 real games to brush the rust off and they will soon be a well balanced team again, considering Allegri is in his 2nd year and will soon be done with his experiments with the team (mostly trying to see if Aquilani can be real alternative at LM, and not just CAM, which he obviously cannot be).
Also, Milan is already prepping their winter mercato, which Galliani considers a key moment in reinforcing the team for the rest of the season, and considering the underwhelming summer mercato, I wouldn't be surprised if Milan is more active in the winter this year.