With the Champions League back in action, it's been a busy period for those involved in the league.

And it was a night of surprises and underdogs as Real Madrid, Manchester City and Bayer Leverkusen all surprisingly ended up on the losing side.

Here's what we can take away from Tuesday's events.

Champions showing a worrying trend

Reigning champions Real Madrid have had a rough few days.

Vinicius Jr. was thrashed 4-0 in El Clásico and then snubbed for the Ballon d'Or. Now, there's a real risk they won't make the Champions League knockout stages after losing 3-1 at home to Milan.

That puts Los Blancos on just six points from four games, with trips to Liverpool, Brest and Atalanta as well as a home tie with Salzburg on the horizon.

What's more, they can't seem to catch a break, having fallen behind in three of their four Champions League games so far.

There's a lot for Ancelotti to think about at the Bernabéu.

Ruben Amorim gets early advantage over Guardiola

All eyes were on Sporting CP's clash with Manchester City after it emerged earlier in the week that Sporting boss Ruben Amorim would be taking the vacancy at Old Trafford.

He'll already be a popular guy on the Manchester City side after his team came from behind to beat Pep Guardiola's team 4-1 with goals either side of half time.

What's more, he ended Manchester City's UCL unbeaten streak, which stood at 26 games after they broke Manchester United's 16-year-old record in their last game.

Could this be an early indication of what's to come in the Premier League?

Celtic making shock charge for the knockouts

After losing 7-1 to Borussia Dortmund on Matchday 2, it looked like Celtic wouldn't be in the running to reach the knockout stages of the competition.

But two games later, they're already on seven points, ahead of the likes of Real Madrid and Milan, thanks to a memorable 3-1 home win over German giants RB Leipzig.

Rodgers' side have four more games to play, against Club Brugge, Dinamo Zagreb, Young Boys and Aston Villa.

Could they make it to the knockout rounds for the first time since 2012/13?

The Arne Slot train rolls on

I don't think anyone could have predicted how well Arne Slot has settled in at Liverpool.

But since taking over from the legendary Jürgen Klopp, he's fit right in, with the only hiccup being the match against Nottingham Forest.

Facing a Bayer Leverkusen side that had only lost twice in all competitions since Jordan Henderson last played for the Reds, they easily beat ex-player Xabi Alonso 4-0. Luis Diaz scored three goals, putting them in first place at the midway point.

People keep expecting them to lose, but they keep defying the odds. They're now top of both the Premier League and the Champions League.

It's getting tight at the top

While only Liverpool have picked up all four points from their opening four games (Aston Villa can join them tomorrow), the race at the top of the table is still wide open.

Sporting CP and Monaco's wins helped them move up to second and third place, respectively, while Manchester City are at the head of a group of nine clubs on seven points, two of which can move up to ten tomorrow evening.

Five more teams are just a point behind, and the second half of the Champions League league stage is shaping up to be pretty intriguing.

Although it wasn't everyone's first choice, the league format could well prove to be a winner.