They have Boateng and Flamini both can replace Gattuso and ambrosini in any given game.
And again depth is the "biggest advantage" that milan have this season (over their past seasons). They have Ibra-Pato-Ronaldinho-Robinho (when in form those attackers are easily among the best in serie A). in the midfield, they have Ambrosini, pirlo, Gattuso, flamini, Seedorf and Boateng allegri can rotate all season with them (to keep them fresh and ready for matches).
Their two CB's (Thiago silva and Nesta) as I've said in my previous post are IMO the best Pair of CB's in the league. Their backup CB's are Sokratis, Onyewu and Yepes not great but an "ok" option when needed. The fullbacks are just decent (no more or less). Abate, Antonini, Bonera and Zambrotta (two for each position).
What allegri has this season that for example Ancelotti lacked in his years with milan is "depth". Allegri has the privilege To rotate among players in every position throughout the season. Something that Milan had been doing so far this season and they're actually doing it pretty well. Add the club's experience (and sometimes gifts from referees) and you'll have a very realistic league winning team.