Midfielder Urby Emanuelson wasted a great opportunity in the first half and Zlatan Ibrahimovic hit the frame of the goal twice from distance after the break.
Antonio Cassano was then brought on to replace fellow attacker Alexandre Pato but the Italy man failed to produce the magic of recent weeks and the Rossoneri only extended their advantage over Napoli in second to five points.
Third-placed Lazio, seven behind Milan and the surprise packages in Italy this term, created little but their continued resilience against the top teams suggests a Champions League place this term is becoming a strong possibility.
Milan's already crippling injury problems were highlighted by centre-back Daniele Bonera having to be replaced by debutant Nicola Legrottaglie, who signed from Juventus on transfer deadline day on Monday.
Legrottaglie was then substituted with an injury late on after being caught by Lazio striker Libor Kozak, who was fortunate to escape a red card given he had already been booked.
Both Legrottaglie and Bonera were taken to hospital for checks on their neck and head injuries respectively.
Napoli head to Chievo on Wednesday looking to profit on Milan's slip while fourth-placed Inter Milan, 10 behind their city rivals, visit bottom side Bari on Thursday.
