Milan have it all to do as they travel to the Vicente Calderon on Tuesday night, battling to overturn a 1-0 deficit and advance to the UEFA Champions League quarter-finals.
If the Rossoneri lose, it could be their last 90 minutes in Europe's premier competition for some time, as their poor domestic showing this season will see them miss the competition next year.
Atletico Madrid were second-best for long periods at the San Siro, but scored a vital away goal through Diego Costa late on.
How will the second leg play out from a tactical perspective?
Possible Formations and XIs
Milan will utilise the 4-2-3-1 formation that has served them so very well under Clarence Seedorf so far, with Mario Balotelli leading the line and Adel Taarabt as well as Kaka behind him.
Riccardo Montolivo is still banned, but Sulley Muntari could return while Andrea Poli could play wide on the right side.
Atletico have a full-strength squad to pick from after welcoming Filipe Luis back from injury. The Brazilian left-back played 90 minutes against Celta Vigo to boost Diego Simeone's hopes of progressing.
The 1st Leg
Milan's performance in the first leg was arguably their finest this season, spraying the ball around with ease, dominating possession for long spells and probing the Atletico defence.
They were forced into an early substitution, as Mattia De Sciglio was replaced by Ignazio Abate, but it didn't disrupt the flow of the game as they continued to feed Kaka and Taarabt outside the box.
"The only thing we were missing was the goal," Kaka told reporters post-match. After hitting the woodwork twice and carving out numerous chances, you can consider him right to say so.
They moved the ball quickly from side to side, concentrated numbers to the flanks in an attempt to overload Atleti's narrow 4-4-2 shape and looked to play off Balotelli. Thibaut Courtois had to be at his very best to keep his side level.
After Kaka hit the bar, it felt like a matter of time before Milan would score. Some rather...robust defending from Adil Rami and Co. was keeping the opposing strike force at bay.
The Rossoneri's relatively aggressive high line quelled many quick attacks and forced Atletico to have possession and work out how to use it. Simeone's men are more reactive and reflexive than most and struggled when they were forced to ask the questions themselves.
Rami and his line also resorted to treading on Diego Costa's toes and ankles to make sure he knew he was in for a rough one. The strategy that kept him at bay for 82 minutes.
However, the Spaniard headed home the winner late on to give his side the lead in a moment when Milan's defence simply, and inexplicably, stopped playing for a second ball.
Atleti's Cautious Approach
The first-leg result leaves Atleti free to play their preferred compact game plan at the Vicente Calderon on Tuesday—and this is not good news for Seedorf.
The defensive, cautious shell Los Colchoneros set themselves up in at the San Siro is likely to be replicated and Milan will be one of the first to break down this stubborn side if they pull it off.
Here's a shot of them bringing the ball out from the back in the second half, moving slowly with seven men in the defensive phase. The full-backs push forward to create passing angles, but the midfielders drop in to make up the numbers.
They pass the ball long and find Costa, who takes it down and stretches the pitch by taking the ball to the sideline. He holds off the defender with ease and waits for his midfield to filter forward, but after fashioning a chance, there are still just three near the box and six total in the opposing half.
Here's a GIF of the work Costa does after taking this ball down and a peek at just how much he is asked to do for his side in the buildup play.
If you give Atletico the chance to keep men back and lean on their striker, they will—and it hurts. Costa is a fiery character and this dueling with defenders Simeone asks him to do gets him fired up with ease.
He attracts double- and triple-teams, opening holes for Koke, Raul Garcia and/or David Villa to exploit and utilise. Costa can split either side. He's a complete, modern, sideline-to-sideline target man.
The No. 1 objective when playing against Atletico is to stop them from using a cautious approach. Unfortunately, one goal down away from home, Milan are powerless in this respect.
Atleti will defend diligently, set up two narrow banks of four and concede ground in the wider areas. That usually means space for crosses galore, but Diego Godin and Miranda are monstrous in the air and rarely lose a headed duel.
The Rossoneri will have to do what no team—that isn't inspired by Cristiano Ronaldo—has so far, and that's break down Atletico's system man vs. man.
If there was ever a time for Kaka to unleash his inner demon...