There's not really any reason to believe that this AC Milan team can challenge Barcelona, but we'll indulge your desire to read a preview.
Barcelona have been upset many times by some combination of teams with excellent players and managers who come up with an excellent tactical plan. AC Milan, at present, do not have either of those things.
Between the weekend's match against Parma and this week's early training sessions, Milan have lost a few key players through injury. They're also without Mario Balotelli, who is cup-tied. Their dynamic forwards and midfielders will either be limited or not dressed at all when they face Barcelona at the San Siro, and it's fair to wonder whether or not this match is worthy of any serious analysis.
Of course, Massimiliano Allegri and Jordi Roura both disagree. Allegri doesn't want his team to give up before the match starts, while Roura doesn't want his to be complacent. There's no denying the huge gap in quality between the two sides in their current state, though, and there's no question that Barcelona is a heavy favorite to win, even in Milan. It's going to take a colossal series of errors from Barcelona and a terrific performance by the Rossoneri for this tie to remain interesting into the second leg.
Xavi has returned from an injury for Barcelona and is nearly certain to dress, but it's possible that he won't start. Adriano and David Villa did not make the trip to Milan. Over the weekend, Adriano picked up a hamstring injury that will keep him out of Wednesday's game. Villa has been in a hospital for over a week with a kidney ailment.
Milan found out on Tuesday that they will be missing Antonio Nocerino and Mathieu Flamini, a couple of solid two-way midfielders who would have been massive assets for Milan. Mario Balotelli is cup-tied, Giampaolo Pazzini is out injured, and Stephen El Shaarawy will have to pass a late fitness test to play.
Projected lineups (left to right)
AC Milan (4-3-3): Christian Abbiati; Kévin Constant, Philippe Mexès, Cristián Zapata, Ignazio Abate; Massimo Ambrosini, Sulley Muntari, Riccardo Montolivo; Kevin-Prince Boateng, Bojan Krkić, M'Baye Niang
Whoever starts in midfield for Milan vs. everyone on Barcelona: Oh dear. That is not an encouraging midfield. Ambrosini's best days passed him by at least five years ago and Sulley Muntari has been dire this year. Riccardo Montolivo has the occasional brilliant match, but without either of Nocerino or Flamini to do some running for him, things could get rough in the center for the Rossoneri. All of Barcelona's players will end up deep in the center on the attack at some point, so there's not even one or two particular players to account for.
Victor Valdes vs. the derp monster: Milan's best chance of getting a goal (or two) is probably a Victor Valdes error (or two). He's generally an excellent keeper, but occasionally makes horrible errors that leave viewers wondering how in the world he became Barcelona's No. 1 and stayed there for so long. Tito Vilanova won't be too broken up if Valdes walks on a free and he gets, say, Marć-Andre ter Stegen to replace him.
Lionel Messi vs. Philippe Mexes and Cristian Zapata: Okay, so these guys can't stop Messi if he's at his best. We can't even pretend this is a legitimate "matchup", but they have to be careful about when they close down. Messi can't be given unlimited time and space, but Barcelona's goals are generally aided by a defender stepping out on Messi too quickly. Messi drops deep to get the ball, a defender chases, that space is opened up and the Blaugrana spot it. It's the typical Barcelona goal recipe. Mexes and Zapata need to avoid giving away those gifts.
Poor Milan. They've done well to get back into a Champions League place in Serie A, but their injuries and the absence of Mario Balotelli is going to make it nearly impossible for them to steal a result. Barcelona win, 2-0.