On Wednesday night we witnessed one of world football's true shocks—Barcelona were shut down, Lionel Messi was shackled and the Catalan outfit as a whole looked toothless.
For all the possession (73 percent) and completed passes (Sergio Busquets and Xavi managed over 120 each), la Blaugrana totaled just one shot on target.
Jordi Roura looked positively stumped as to how to overcome a gargantuan effort from the Rossoneri, and for an in-depth analysis on how Massimiliano Allegri masterminded a tactical victory, click here.
In the aftermath of the upset, let's ponder what options Barca have in turning this tie around and overcoming elite Italian defending.
The first thing you'll notice is how deep Milan played.
Massimo Ambrosini, Sulley Muntari and Riccardo Montolivo blocked off the middle of the pitch and stayed just five yards in front of the defensive line for long periods.
Allegri played a flat back four with almost no vertical movement from the full-backs, making this a true defensive seven. Barca were always going to have to come up with something extraordinary to pass their way through this.
After 45 minutes, it was pretty clear a change in strategy was required for Barcelona to score. Unfortunately, Roura made hapless changes that, if anything, hindered rather than helped his side.
Alexis Sanchez came on to ghost around in something approaching the No. 10 role, which forced Messi even deeper. Pedro was consistently ignored despite his desperation to breach the channel between Philippe Mexes and Kevin Constant and Andres Iniesta offered very little other than a few nice turns.
What Roura needed to introduce was width.
Iniesta played "from the left" not on the left, and the direction of his dribbling was always to the inside. Jordi Alba struggled to get into a position to overlap, while Dani Alves was also pretty subdued.
The heat maps, provided by the Sky Sports Events Centre, are testament to how little progress key players made.
Iniesta and Messi simply floated in no-man's land, while at no point did Alves find the byline.
This simple lack of width was a serious problem, and with the middle so heavily congested there was absolutely no way through; Barca often face this in away games when the pitch is purposefully marked out at the minimum regulation width, so there should have been a plan to deal with it.
La Blaugrana faced a similar problem in last year's UEFA Champions League at Stamford Bridge.
Although Chelsea were nowhere near as organised and took more of a "let's stack the box and hope for the best" mantra, they stifled Barca by packing the middle of the pitch.
For the second leg at the Nou Camp on a far wider surface, Pep Guardiola threw Isaac Cuenca (who was very, very new to the first team then) into a starting role and instructed him to stay touchline-wide on the left flank.
This widened the Blues' basic shape as Branislav Ivanovic could not tuck in alongside his central defenders on a consistent basis and was frequently dragged out to close Cuenca down.
With a player on each touchline (Alves was on the right), Chelsea had no choice but to spread, or else they would be allowing cross after cross to come in and threaten Petr Cech.
Sergio Busquets' opening goal came from exactly this—a surge down the left, low cross and finish.
So where was that on Wednesday? Cristian Tello, a player very capable of doing exactly what Cuenca did against Chelsea, remained on the bench as Sanchez entered the fray and contributed very little.
The only way to beat a tight unit is to stretch it horizontally or go for an aerial assault. Barca can't do the latter, so the former is the only choice.
The heat maps show key players like Iniesta and Messi got nowhere near the penalty area, yet they carried on knocking for the entire 90 minutes—an astounding lack of adaptability.
Whether it's Roura or Tito Vilanova in charge for the second leg, they simply have to provide some true width.
Look at how effective Thomas Mueller is for Bayern Munich when he hits the right byline and pulls it across—he creates utter chaos that can rip apart even the soundest defensive structure.
There's hope for Barca in the second leg, but only if they make the necessary changes in personnel. Get Tello and Alves to the byline, suck one of Milan's midfield three out wide and create yourself two yards of space.