Milan played with 4-2-3-1 line up. And Chelsea played with 3-4-2-1 line up.
In set attacks, Milan were like 3-2-5. Normally if attacking team uses 5 forwards, in the set attack, attacking team can play more directly. It’s because if defending team plays with 4 defenders formation, attacking team will have 1 free player. But also if we think this 1 more player will be in one of the wings, defending team doesn’t need to cover him until the ball goes to this extra player which is now in the wing.
It’s because if defending team has 4 defenders and they meet the opponent in mid block or in front of their own penalty area, like how Chelsea met Milan, first of all, defenders have to cover central spaces. We have 2 half-spaces and also we have to divide centre to 2 spaces (because it’s too big to count as one space). So we have 4 central spaces. If every defender would cover 1 space, then there wouldn’t be any player for the wing. But it’s not a problem because for now wing is not important to cover. Because now attacking team is not closer to defending team’s goalpost. If attacking team would be closer to the goalpost, when the ball goes to wing, defending team’s players have to cover wing quickly too. Because now from any spaces closer to goalpost, attacking team can be dangerous. When the ball goes to wing, if defending team can’t cover wing quickly, at least, attacking team would find a free crossing opportunity.
Also of course attacking team could use this extra player in one of the central spaces with a player which is already in this space. For example they can take position back to back like as you can see in this picture. This positioning would help the attacking team’s extra player who doesn’t match with one of the defending team’s defenders. Because in this player’s space, already there is an attacking team’s player who matches with the defending team’s one player. So in this space, defending team’s player can’t cover this extra player. And also now because defending team’s all defenders match with an attacking team’s forward players, non of them can cover this extra player. In this situation, defending team has to be supported by midfielders. If at least one midfielder would take position closer to this extra player, defending team can cover this extra player.
If we go back to our situation, this positioning didn’t work for Milan against Chelsea. Because Chelsea had 5 defenders. Because 5 defenders could match with 5 forwards directly, Milan couldn’t raise the tempo as they wanted. And this time, because in everywhere there is a numerical balance, Chelsea became the team which could benefit from this situation. Because Milan didn’t have the same player quality like Chelsea.
And also as you can see in the last picture, when Milan tried to make game, naturally, Theo Hernandez and Bennacer took position between Chelsea’s forwards and midfielders. And also again naturally, Milan’s stoppers took position between Chelsea’s forwards and Milan’s goalkeeper. These positionings caused extra problems for Milan. Because when they try to play direct, they lost balls quickly after trying to send balls to forward players. And when they lost balls in Milan’s forward players’ areas, because Chelsea’s midfielders were closer to Milan’s forwards then Milan’s midfielders, Chelsea’s players could take the second balls comfortably and started transition attacks. Also after Chelsea’s midfielders took the second balls, because Chelsea’s forward players were closer to Chelsea’s midfielders then Milan defenders, Chelsea’s midfield players could pass the ball to Chelsea’s forwards directly.