As long as Ambro and Slowdorf along with Bonera are not on the pitch together, Milan will be fine.
I think by the time of the first leg, Abate, Boateng and Nesta will be ready. Having a back four of Abate, Nesta, Slva and Antonini will do against Barca. Then insert Mexes as DM in front of the defense with Nocerino and Urby(mainly due to his speed in tracking back). With Boateng and Robinho pressing on both sides and Ibra being in the middle.