The Champions League is back. With four games on the slate this week, let’s take a quick around a few salient statistical things likely to be happening on the pitch.
Atlético Madrid v Liverpool
Atléti hasn’t struggled as much this season as the narrative surrounding them might suggest. And while Liverpool are one of the favorites to win the Champions League (again) their performances also haven’t quite lived up to their otherworldly return (though, really, could any team in history have ever gone on such an extreme winning run as Liverpool without a nice heaping dollop of luck scooped on top of a truly great team?). In theory, this match should be a closer one than it might seem on paper. Digging into the stats, however, shows two teams whose numbers line up in a curious way. One of the mysteries of Liverpool’s season has been just how good they’ve been at set pieces. From direct and indirect free kicks, as well as throw-ins and corner kicks, they've scored 12 goals from 7.04 expected goals. 
Again, in theory, it's true that tight execution and well-conceived set-piece design can let a team run a bit ahead of expectation on set pieces, but figuring out exactly how much of Liverpool’s achievement is an accurate reflection of execution versus a sprinkling of good fortune is devilishly difficult Similarly, even if we attribute a slice of those goals to being excellent at what they do, historically speaking, it's just a matter of time before defenses adjust and Liverpool’s edge will have to be generated anew from different strategies. Most years, you’d expect this Liverpool strength to be countered by Atlético Madrid’s own strength. After all, a big rugged defense and strong set-piece defending are Diego Simeone's calling card. However, that's not been the case this season. In fact, Atléti have conceded nine times from set pieces this season. On the one hand, that number isn’t quite as bad as it seems. They’ve only conceded 6.86 xG, so there’s some bad luck hovering above them (though even that 6.86 figure is a disappointing total for a Simeone side). 
On the other hand, a whole heck of a lot of that bad luck seems to fall directly at the hands (and feet, and incredibly long arms) of superstar keeper Jan Oblak. Despite his formidable track record, Oblak is having a relatively poor season — he's allowed two more goals than an average keeper would given the shots on goal he’s faced. 
Break out the set pieces and something even more surprising jumps out. Oblak is his usual stellar self during open play, but he’s conceded a whopping 4.29 goals more from set pieces more than expected from an average keeper. How much of that is a true reflection of his poor play and how much is a knock-on effect from a team struggling to defend is up for debate. Either way, it’s an eye-popping number 
Ultimately a match with two strong defensive sides means this tie could well come down to set pieces. And, if so, Liverpool will likely have the edge.
Borussia Dortmund v Paris Saint-Germain
These two teams are excruciatingly difficult to analyze in the proper context. Stats rely on aggregates and there are reasons to believe the aggregate totals of both PSG and Dortmund are not necessarily an accurate reflection of where they are in the current moment. Dortmund have recently undergone a tactical evolution. More than ever they’re playing like a prototypical Lucien Favre side, mixing conservative possession with moments of quick attack into space. It also helps that they have the best two teenagers in the world in Jaden Sancho and Erling Håland, both of whom are playing with the amp turned up to 11. Since returning from the midseason break, Dortmund have yet to score fewer than three goals in a match, though they’ve also faced relatively easy opponents. Their only match against a side in the top half of the table was a 4–3 loss away to Bayer Leverkusen. The question is whether Dortmund will be able to maintain their improved attacking statistics against top-level competition or whether the numbers of the last six weeks can be attributed to a weak schedule. If they can exploit the openings PSG will likely leave them, then occasionally they'll manage to hang with the favored side. 
PSG, on the other hand, face the same problem season in and season out. They are simply so much better than other Ligue 1 teams that it is impossible to get a true impression of how they will fare against stronger competition. Dortmund are at least the second-best team PSG have played this season (depending on how you interpret Dortmund’s form now and Madrid’s form at the beginning of the year, arguably they’re the best). On the other hand, as always, PSG has Kylian Mbappé putting up these numbers, 
And Neymar putting up these numbers. 
Thomas Tuchel runs a team that wants the ball, needs the ball and uses the ball in midfield to create great chances for the side’s superstars in the box. They aren’t obsessed with taking a lot of shots, but they are obsessed with taking the best ones. 
If things go wrong it will be because Dortmund manage to thwart that plan by counterattacking into the space that PSG’s extensive and highly aggressive possession leaves available. If PSG’s domestic season is any indication they will be just fine. This attacking radar simply outweighs any concerns. 
The problem is that it's simply been so long since PSG has faced competition the caliber of Dortmund that it's hard to weigh how meaningful that awesome radar is. While PSG's numbers suggest Dortmund shouldn't beat them, it's quite possible those numbers aren't reliable.