Or what I think Allegri learned from the first leg1) Messi can be frustrated
One of the advantages of Allegri's midfield diamond is how it forces Messi to deal with additional midfielders when he tracks back, which he will do when he isn't immediately presented with a chance. While this does not nullify him, it does present additional obstacles. Messi is absolutely lethal in his false nine role but this ultimately depends on the centerbacks moving up to create room. Crowding the midfield (see: El Shaarawy) creates less gaps in the back and makes the midfield a numbers game. A diamond with a destroyer like Ambro allows Milan to remain narrow and reduce the number of defense-splitting passes Barcelona can play. There aren't many teams better at playing those passes after all.
With a narrow backline, you get Xavi receiving the ball with his back to goal, with the added discomfort of Ambro on him. If Xavi can't see forward, he's not nearly as good as he can be.
Remember the ninja-nesta foul? Look at where Messi started his run v. where he started his runs earlier on in the game. The defenders didn't chase forward but tracked back without losing shape. Against most teams, you'd find two things: loss of shape and a more advanced Barcelona player on the wings to counter the wide mids.
2) Long balls are fine. Indeed, they're the best.
Hitting long balls to Zlatan is a good thing. Here's why. Barcelona like to dispossess the opposition in their own half, allowing Messi to start his runs closer to the penalty area. However, if you hoof it forward, even if you lose the ball, the counter-attacking phase of Barcelona's play (arguably their most attractive/effective) starts further away.
Yes, it's not the most romantic thing in the world. But if you were looking for romance, you were supporting Barcelona anyway. Or you're Berlusconi. For my part, I find Ibra's takedowns to be one of the most graceful things in the game so I'm happy. Aside from Pique, Barcelona simply don't have a single player who can compete in the air. Even the otherwise tall Busquets is just plain bad in the air.
3) Seedorf is key
Against a team like Barca (or Dortmund or another such), someone who can really hold the ball is key. We're not a counter-attacking team with a Lavezzi and we're not trying to be. Aquilani, much as I like him, cannot do that in such narrow spaces as Seedorf. Yes, he's slow. He can be bypassed. But making tackles and being the midfield dynamo is not what Allegri puts him out there for so those criticisms are moot. Outside of Ibra, no other player at Milan can withstand being crowded like he can. This means he's at liberty to play passes like that beautiful one to Ibra, the one that should have been buried. What stymied Dorf was Robinho. Robinho was always in no man's land. I think Allegri will look to El Shaarawy to exploit space.
Barca's wingback, Alves (if you'll notice, Milan just let him run; also, Milan seem to be pushing Barca to play through him via Seedorf) bombs forward. Someone like El Shaarawy who can run for days will make him think twice about making his runs. He can also move to the other wing to exploit space behind Tello. Is that going to stop Alves making his runs? Probably not. If you'll look at Alves' crosses, they're not particularly threatening unless they're in behind the defensive line. With a deep backline, that's not much of a risk. Ultimately, if we're going to have to sub an attacker, I hope he's a left-sided one to account for space behind Alves.
4) They're afraid of Boateng
After his stunning goals against them, I'm beginning to think Barca don't really want to take him on. While he wasn't especially good in the first leg, we bought a lot of purchase simply based on how intimidated Barca was by him, especially in the first 20 minutes.
I don't expect Milan to play that much differently than last week, aside from adjustments to account for Cesc/no Xavi. To be honest, I don't quite know what the purpose is of noceringhio against good opposition. That's something I'd love for Allegri to qualify. Against weaker teams, there's no one I want more in the midfield but against Barcelona, I feel he's not very useful. I'd like Muntari or Strasser there but neither of them can play so bleh. Ibra will score. I look forward to it, and our (hopeful) victory. As someone who's followed Ibra's career for years (even when he was at Juve, who I dislike), I would dearly love for him to get sweet revenge on the Philosopher.
This reminds me of 1994. That was one of my first seasons watching. I feel it in the air.