Ten things we learned during matchday four of the Champions League.

The Champions season returned to center stage as the season reached its midway mark for those competing.

And it was a night of surprises and underdogs, as Real Madrid, Manchester City, and Bayer Leverkusen all unexpectedly lost.

Champions show a disturbing trend.

Real Madrid, the reigning champions, have not had a nice few days.

Vinicius Jr was overlooked for the Ballon d'Or after losing 4-0 in El Clasico. They now face a serious risk of missing out on the Champions League knockout rounds after losing to Milan 3-1 at home.

It leaves Los Blancos with just six points from four games, including trips to Liverpool, Brest, and Atalanta, as well as a home match against Salzburg.

Worryingly, they continue to fall behind in three of their four Champions League games.

There's plenty for Ancelotti to think about at the Bernabeu.

Ruben Amorim gains an early advantage over Guardiola.

All eyes were on Sporting CP's encounter with Manchester City after it was announced earlier this week that Sporting head Ruben Amorim will take over at Old Trafford.

And he will already be a popular figure in Manchester's Red Half, where his team came from behind to defeat Pep Guardiola's side 4-1 with goals either side of halftime.

More crucially, he interrupted Manchester City's UCL unbeaten streak, which had stood at 26 games since they surpassed Manchester United's 16-year-old record in their previous encounter.

Is this a preview of what's to come in the Premier League?

Celtic make a stunning drive for the knockouts.

After being crushed 7-1 by Borussia Dortmund on Matchday 2, Celtic had little prospect of making it to the competition's knockout stages.

However, two games later, they have seven points, ahead of Real Madrid and Milan, thanks to a stunning 3-1 home triumph over German giants RB Leipzig.

In their final four games, Rodgers' side will face Club Brugge, Dinamo Zagreb, Young Boys, and Aston Villa.

Could they reach the knockout stages for the first time since 2012/13?

The Arne Slot train continues on.

Few would have guessed how well Arne Slot has adapted to life in Liverpool.

But since taking over from legendary manager Jurgen Klopp, he has blended in smoothly, with his sole blemish coming against Nottingham Forest.

Faced with a Bayer Leverkusen side that had only lost twice in all competitions since Jordan Henderson's last appearance for the Reds, they romped to a 4-0 win over ex-player Xabi Alonso, with Luis Diaz scoring a hat-trick to put them top of the table at the halfway stage.

People keep expecting them to lose, yet they continue to defy the odds and are currently leading both the Premier League and the Champions League.

Could it be their year again?

It's becoming tight at the top.

Though only Liverpool has picked up maximum points in four games (Aston Villa can join them tomorrow), the top of the standings remains quite tight.

Sporting CP and Monaco's victories propelled them to second and third place, respectively, while Manchester City leads a group of nine clubs tied on seven points, two of which can reach ten points tomorrow evening.

Five more teams are just a point behind in what promises to be an exciting second half of the Champions League league stage.

Though initially unpopular, the league model could prove successful.

Reborn Barcelona might go all the way.

Barcelona's performance in the Champions League knockout stage has served as somewhat of a cautionary tale.

Exits to Roma, Liverpool, and Bayern Munich left blemishes on the Blaugrana's record that continue to plague fans to this day.

 

Under Hansi Flick, however, they have emerged as a force to be reckoned with on the continent's greatest stage, comparable to LaLiga.

A 5-2 defeat of Red Star proved to be the latest example, as they drew closer to securing an automatic qualification spot.

They could still fall short, but Flick's methodical approach indicates that Barça's days of bottling it are finally over.

Luck is no longer on Arsenal's side.

Mikel Arteta must wonder if Arsenal is officially cursed after their start to the season.

The Londoners are already lagging behind in the Premier League title chase, and their luck appears to be running out in Europe.

Despite dominating possession, they fell short against Inter and were undone by sliding doors in the first half.

Mikel Merino was the fall guy on both instances, being denied a penalty by Yann Sommer's punched clearance before giving one away himself.

Despite Martin Ødegaard's long-awaited return to play, the Gunners failed to produce a late attack at the San Siro.

PSG risks being an early casualty.

The Champions League was always going to need an early victim to justify its new design, and it may soon have one.

Paris Saint-Germain has only scored maximum points once in their first four games and risks missing out on the knockout rounds.

 

With Atlético de Madrid's late smash-and-grab victory, Luis Enrique's side is currently just four points from the play-off spots.

Les Parisiens have not failed to advance to the next round since 2004/05, and their upcoming matchups do not look promising for avoiding a repetition.

With games against Bayern Munich and Manchester City on the horizon, the Ligue 1 giants may fall out of contention entirely.

Lookman inspires Atlanta again.

Ademola Lookman, one of European football's top prospects, could be on his way to Atalanta.

Following his Europa League victory in Bergamo last season, the Nigeria international continues to score goals.

A smooth finish to open the scoring in La Dea's away triumph over Stuttgart brought his tally to eight goals from 12 this season.

Gian Piero Gasperini's team has a history of overcoming the odds, and a favorable schedule provides them an extra advantage in qualifying.

The essential seven points from their trip to Young Boys before hosting a lackluster Real Madrid and Sturm Graz is certainly attainable.

Brest continues to fly the flag.

After four games, few would have predicted Brest to be ranked higher than Inter, Barcelona, and Borussia Dortmund.

However, Ligue 1's surprise gift is continuing to do things their way, winning 2-1 against Sparta Prague to prolong their unbeaten run.

It remains to be seen whether they will be able to continue their fairytale run with a definite knockout round spot.

Les Pirates' journey to Barcelona later this month does not look promising, but they will still be optimistic about their chances of progressing.

Football fans enjoy a good underdog, and Eric Roy's team might be a genuine feel-good tale among the heavyweights.