Few clubs came into this season with grander ambitions than Real Madrid and Paris Saint-Germain. Both clubs were among the top contenders to win the Champions League when the competition began. Now, one will be eliminated from competition after the two face off Tuesday in the second leg of their round of 16 matchup. And the problems run deeper than just elimination — PSG is looking to survive an injury to its superstar Neymar, and Real Madrid has already fallen out of title contention in La Liga. Whichever team fails to advance from the match will have questions to answer about what went wrong.So how did we get here? Let’s start with the summer transfer window after the 2016-17 season. Real quite reasonably avoided making major changes to its roster after winning three of the last four Champions League trophies. PSG, having once again been eliminated from the Champions League before the semifinals, embarked on an unprecedented spending spree — laying out hundreds of millions of dollars to acquire Neymar from Barcelona and Kylian Mbappe from Monaco.Despite these purchases, PSG is once again struggling in the Champions League. A 3-1 first-leg defeat in Madrid has left the French with a roughly 25 percent chance of advancing to the quarterfinals, according to FiveThirtyEight’s club soccer predictions. To reach the quarterfinals, PSG must win by three or win by two while holding Real scoreless. If PSG wins 3-1, the teams will be tied in both aggregate and away goals over the two legs, and the match will go to extra time and possibly penalty kicks to decide who will advance. For PSG, the Champions League appears to be slipping away. And worse, Neymar recently broke a bone in his foot, which will sideline him for at least six weeks.For both ball progression through midfield and shot creation around the penalty area, PSG runs through Neymar. He has scored 19 goals and assisted 13 in Ligue 1, with a non-penalty scoring and assist rate of 1.34 per 90 minutes played. These numbers are no fluke: Neymar hasn’t benefited particularly from good fortune in either his own or his teammates’ finishing. Neymar has taken shots valued at around 13 expected goals, a statistical estimate of the quality of scoring chances, and created chances worth about 11.5 expected goals. His expected goals and assists rate of 1.18 per 90 minutes is the highest of any player in the top five European leagues1The English Premier League, German Bundesliga, Spanish La Liga, Italian Serie A and French Ligue 1. (that means he’s doing better than Lionel Messi).Neymar also moves the attack forward through midfield or into the penalty area before the shot more effectively than anyone else in the top leagues. He has provided about 7.7 progressive passes and runs per 90 minutes, just edging out Messi, who has provided 7.2. (Progressive passes and runs occur when a player either moves the attack forward by 10 or more yards beyond its furthest point of progression or moves the ball into the penalty area for the first time in a possession. A progressive run must also include a successful take-on of an opponent.)These comparisons to Messi are not superfluous — Neymar’s production at PSG has reached such a high level that Messi is his only peer. This chart shows the company that Neymar is keeping this season. It displays all 9,000-plus player-seasons in the top five leagues in which a player had at least 1,500 minutes. There are 10 seasons in the top right corner, where players have more than 0.9 expected goals plus expected assists per 90 minutes and more than five progressive passes and runs per 90 minutes. Eight of them are Messi, starting with his 2010-11 season and running through the current one. And nestled among them is Neymar’s current season. This analysis might suggest that Paris Saint-Germain should abandon hope — the attack has been totally dependent on Neymar, and now he’s out. However, PSG may be the only club in the world that is rich enough to have a reasonable Neymar replacement sitting on the bench. Angel Di Maria, who played in Neymar’s wide forward position this weekend against Troyes, put up similarly huge numbers for PSG in 2015-16 (before Neymar’s arrival). While that season was a career-best for Di Maria, he has consistently put up more than 0.5 expected goals and assists per 90 minutes (averaging over 0.7) and 4 to 6 progressive passes and runs per 90 minutes. Most clubs would have to reshuffle their tactics upon losing Neymar, transferring some of his scoring load to one player and some of his ball progression work to others. But PSG can plug in Di Maria and keep everything else roughly the same. If PSG is to get the big win it needs on Tuesday, the man in the center of it is likely to be Di Maria.Scratching back from a two-goal aggregate Real Madrid lead might normally seem like an insurmountable task, but Real hasn’t been the same team this season. Los Blancos have already fallen 15 points off Barcelona’s pace at the top of the La Liga table, effectively conceding the league title with months to go in the season. Real’s defense has been unusually permeable this season, conceding 29 league goals. That’s the club’s worst defensive record through 27 matches since 2010-11. Real has conceded 43 clear scoring chances — defined as a situation like a one-on-one in which a player is expected to score — also its highest number since 2010-11.On the attacking side, the team appears to be reeling as well — but these numbers are a little misleading. Typically, the very best teams in the world outperform expected goals. After all, these teams have the best strikers, and the best strikers convert the easy chances and also score more often in situations when goals are not expected. Real Madrid is getting their normal amount of chances this season: The team has 63 expected goals and has averaged 61 through 27 matches over the past seven seasons. What has changed then is that the finishing has gotten worse. This year, Real has converted only 58 nonpenalty goals from those chances. Cristiano Ronaldo, who has been uncharacteristically unproductive with his chances, accounts for most of the gap, with 13 nonpenalty goals and 17 expected goals.These statistics provide the context for Real Madrid’s disappointing position in La Liga. The weakened defense is enough to make Real fall short of first-place Barcelona, but it is uncharacteristically poor shooting from superstars like Ronaldo that has dropped Los Blancos out of title competition entirely.The stakes could not be higher for this match. PSG is desperate and missing its star. Real Madrid appears more vulnerable than usual — even if the attack is most likely better than its relatively disappointing top-line numbers show.Neither team has much to play for this season other than Champions League glory, and both clubs were constructed precisely for this task. The stage is set for a great European clash between two of the richest and most successful clubs — even if it’s not what these two heavyweights envisioned when this all started.Check out our latest soccer predictions.