Champions League tactical battles: What each team needs to advance
Michael Cox looks at the key tactical questions facing the Champions League quarterfinalists ahead of the second legs.
Barcelona vs. Juventus: Which system does Luis Enrique use?
Barcelona's formation has been a constant source of debate in recent weeks. With no specialist right-back to call upon -- their former regular Dani Alves now plays for the opposition here -- Luis Enrique has been forced to devise complex systems to get around the problem. This has often involved Barcelona playing a hybrid between a three and four-man defence, with poor Sergi Roberto forced to drop in and cover the natural right-back zone. This is particularly tough against Juve, who field a natural centre-forward, Mario Mandzukic, on their left.
This caused Barca all kinds of problems in the first leg, with Juve continually switching play from flank to flank to exploit the space on the outside of Barca's back three before Roberto could retreat into the back four and cover the space. Their goals were extremely similar, stemming from attacks down the flanks and then finding Paulo Dybala, who curled home two lovely goals.
Barca must surely deploy more of a four-man defence this week -- especially because a single concession will leave them needing to score five goals, and while they've already demonstrated their capacity for astonishing comebacks in this competition, five goals against this Juventus side is surely beyond them.
It remains to be seen who will be fielded in that right-back role. With Sergio Busquets returning to his holding midfield position, there would be some logic in using Javier Mascherano there, at least in the opening stages before Barcelona need to go all-out attack. After last week's disaster, however, Luis Enrique needs to recover some of his reputation as a good tactician with some kind of intelligent solution.
Monaco vs. Dortmund: Can Monaco shut down the tie?
Although understandably overshadowed by the attack on Dortmund's team bus, the contest between these two young, energetic, attack-minded sides last week produced a brilliantly open game and five goals, with Monaco taking a slender lead back to the south of France. Now, their task is to protect that advantage.
For all Monaco's attacking brilliance in this season's Champions League, their defensive record is very poor. They've only kept one clean sheet in Europe this campaign, the 3-0 home win over CSKA Moscow -- who are notoriously poor on the road -- and they managed to concede three goals away at Bayer Leverkusen and five at Manchester City. They're reasonably good at protecting the defence with hard-working midfield players, but the full-backs fly forward and leave spaces for opponents to counter-attack.
To triumph in this season's competition -- probably the most open the Champions League has been since 2004, when Monaco reached the final -- they'll need to show more tactical intelligence and more defensive solidity at some stage. Left-back Benjamin Mendy has been sensational going forward, but can he remain in a proper defensive position and deny Dortmund's quick attackers space? Can Fabinho resist his forward surges, anchor the midfield and help to see the game out with simple short passes?
Playing more reactively may also help Monaco cause Dortmund problems; Thomas Tuchel's three-man defence was repeatedly exposed by quick moves last week and the speed of the sensational Kylian Mbappe is best in counter-attacking situations. If Monaco show maturity and intelligence here, there's no way they can't go all the way.
Real Madrid vs. Bayern Munich: Can Lewandowski provide the difference?
Bayern Munich's preparations for the first leg were thrown into chaos when it emerged that a shoulder injury would keep out Robert Lewandowski, their prolific striker from Poland. Although Thomas Muller is hardly a disastrous replacement, he endured a poor match in Munich and was substituted by Carlo Ancelotti even at a stage when Bayern were behind and chasing the game.
Lewandowski will return at the Santiago Bernabeu, however, and this will be a perfect demonstration of quite how transformative his presence can be. With him up top, Bayern are better in so many ways -- they can play more directly, cross the ball more and they are also more effective at playing passing combinations in dangerous positions on the edge of the box. Lewandowski's touch and link play are somewhat underrated because he's considered a pure No. 9, but his creative skills shouldn't be underestimated.
This may prove crucial, because Real Madrid are often guilty of not protecting their defensive quartet effectively. Casemiro has rightly established himself as a fixture in the holding role, but if Real are overloaded between the lines, the centre-backs can drift out of positions too easily, creating gaps for others to sprint into. Watch for aggressive forward runs from Arturo Vidal and Thiago Alcantara, who know they're more likely to receive clever passes from Lewandowski than Muller.
Lewandowski confirmed his status as a top-class centre-forward against Real four years ago, when he smashed in all four goals in a 4-1 Dortmund victory at the semifinal stage. If Bayern manage to complete a dramatic turnaround here, Lewandowski needs a performance of similar quality.
Leicester vs. Atletico Madrid: Which side gets chances to counter?
The meeting between these two sides was always likely to be the most cautious and defensive-minded Champions League quarterfinal tie. Leicester essentially took their title-winning template from Diego Simeone's Atletico Madrid, using the same ultra-compact, counterattacking 4-4-2 formation that is so effective at upsetting possession-based opponents.
In a clash between two counterattacking teams, however, the first leg was somewhat dreary. There was only one goal, Antoine Griezmann's penalty, and that stemmed from a counterattacking chance created when Leicester pushed men forward for a set piece. Otherwise, both sides concentrated on denying the other space.
Expect something similar this time around. Atletico can play defensively in the knowledge that a clean sheet will see them progress, but Leicester must also determinedly guard against conceding. One Atletico goal will mean Leicester will need to score three, and very few sides manage that against Simeone's team.
This will therefore probably be a very similar contest to the game in Madrid: two sides sitting back and proactively disrupting their opponents' best form of attack. But sooner or later, Leicester will need to push forward and take the game to Atletico, which will then leave spaces for their speedy forwards to sprint into. Throughout Leicester's incredible run of form over the past 18 months, they've very rarely approached big matches knowing they need to win, and that challenge might prove too difficult against Europe's most ruthless counterattackers.
Michael Cox is the editor of zonalmarking.net and a contributor to ESPN FC. Follow him on Twitter @Zonal_Marking.