Champions League by the numbers: Four trends to keep an eye on during Matchday 4

With Champions League Matchday 4 already upon us, here are four important numbers to know going into all the action (which you can catch on CBS All Access this week).

Gerard Pique's injury on Saturday against Atletico Madrid means Barcelona have precisely one healthy center back heading into Tuesday's match against Dynamo Kiev. So, everybody wish good luck to French defender Clement Lenglet. Even with a mostly healthy, or at least healthier, team, Barcelona still haven't been strong in defense this season. They're conceding 1.13 goals per match in La Liga, which ties them for an exceedingly average 10th in the domestic league. The fancier numbers don't pay a much rosier picture. Ronald Koeman's team is conceding chances worth 1.21 expected goals on average per match, which is only the eighth-best total in the league.

Barcelona's struggles have yet to catch up with them in the Champions League where they've conceded only two goals in the opening three matches. Only the Premier League trio of Liverpool, Chelsea and Manchester City have conceded fewer. But, the underlying numbers tell a different stories. The Catalan side has been extremely mediocre in their opening three games, allowing opponents chances worth 1.51 xG per match, which is only the 17th best in the Champions League. It's less that Barcelona have been a better defensive team in the Champions League than at home in Spain, and more that their opponents haven't punished them for their lapses. All of this may not matter against competition as mediocre as Kiev, but in a season where Barcelona are desperate for silver linings, fans might have hoped their defensive record in the Champions League would provide one. Sadly it doesn't stand up under scrutiny.

Chelsea seem to have fixed their gaping problem at goalkeeper. Since he's arrived and taken the starting job from the woeful Kepa Arrizabalaga, Mendy has made 15 saves and conceded exactly once in 720 minutes of combined Premier League and Champions League minutes. The 16 shots on target he has faced had an expected shots on target value of 3.20, so the fancy stats suggest he's single-handedly saved Chelsea over two goals worth of value since his arrival, an important turnaround for a team that was struggling so desperately in goal.

Fixing the keeper problem has also led to Frank Lampard building a much more defensively solid team than he had last season. In the Premier League, Chelsea are now conceding the fifth fewest shots at 10 per match, and the fewest xG per match at 0.86. It's a strong defensive record that suggests that Mendy has done more to shore up the defense than simply save some goals, but rather with a competent net minder, Chelsea's entire defensive structure has improved. The result is a side that might be renowned for all the attacking talent it acquired this summer, but is even more notable for the defensive strides it's taking.

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Atletico Madrid being good is nothing new. Atletico Madrid being a dynamic attacking side, however, that's a trick Diego Simeone's team has never pulled off before. So far this season, however, they're one of the most potent attacking sides in Europe. Simeone's Atleti have 18 goals, the second most in La Liga. The league leaders, Real Sociedad, have needed two more games to score 21. Nobody in Spain tops Atleti's 2.25 per match.

The slightly fancier stats are also kind to Atleti, though perhaps not quite glowing. The team's xG total of 1.77 per match suggests that Simeone's men have been on a hot run of form that is unlikely to continue forever, but even that 1.77 number is third best in the league. The team takes 13 shots per match, the third most in the league, and, crucially for a team that has spent many years scraping together goals from set pieces, Atleti are getting 3.63 shots from open play on target per match, second most in La Liga.

Put all those numbers together and Atleti start to look like a team that is finally adding attacking dynamism to its usual stalwart defensive posture. While the performances will likely come down to earth a little bit, given how good Atleti's defense is, this team might be able to contend for titles on multiple fronts this season.

Neymar is having a truly bizarre season for Paris Saint-Germain. A look at his underlying numbers shows him to be the dominant force he's always been, albeit one who is frequently too fragile to play heavy minutes. His 1.15 xG per 90 minutes (excluding penalties) is first in Europe's big five leagues among all players who have played at least 300 minutes. He's taking 5.32 shots per match which is second in the top five leagues. Also, unsurprisingly for Neymar, he leads the big five leagues in attempted take-ons per 90 minutes at 11.80 (he's one of only two players attempting more than nine per 90). And lest you think all of this means he's being a selfish ball hog, he's also averaging 26.61 passes made in the attacking third per 90, the sixth most in Europe's top five leagues. All of which is to say, the numbers suggest he's doing a whole heck of a lot right.

So, what's the problem? Well, the problem is that if you don't look at the fancy numbers, in seven combined matches between Ligue 1 and the Champions League he has two goals and three assists. Those aren't bad numbers, but they aren't superstar numbers. And when you're Neymar, and you play like Neymar, and you act like Neymar, and you miss matches like Neymar, putting up fine but not superstar numbers simply isn't gonna cut it. So, who's the real Neymar, the guy with the great underlying stats or the guy with the merely pretty good goals and assists? Well, the smart bet is that eventually the underlying numbers predict where the goals and assists are headed. The problem for PSG and manager Thomas Tuchel is that with a must-win game looming this week -- and an increasingly toasty hot seat -- Neymar's production really needs to hurry up and get here.

CBS Sports research and TruMedia assisted with this article.