We've reached the serious end of the group stages on Paramount+. There are five teams -- Bayern Munich, Red Bull Salzburg, Juventus, Liverpool and Ajax -- that can qualify for the round of 16 this week while a host of other sides are fighting just to stay in the competition. Here's what to keep an eye out for.
Juventus vs. Zenit: Claudinho ends Juve win streak
In what is threatening to be a meltdown season for Massimiliano Allegri and Juventus, the Champions League is the great lead box that, for now, is keeping its inhabitants safe. Three games have brought three wins but we should start with some swift caveats. Malmo may just be the worst team in the competition while the Chelsea victory stands out for all the wrong reasons. It was one of the precious few games where they looked like the defensive force of old, serving to juxtapose just how leaky they've been at the back the rest of this season.
Meanwhile, in Serie A they have lost as many games as they have won and have conceded as many goals as they have scored, although they have 3.3 more expected goals (xG) scored than conceded in their ledger. A team that is losing to the likes of Verona, Sassuolo and Empoli has to expect their travails to bleed into the European game eventually. Why not against a Zenit side that has given as good as they have got through three games so far? Both Chelsea and Juventus had almighty problems getting three points against the Russian champions. And since Zenit's defeat to the latter they have proceeded to smash seven past Spartak Moscow and four against Dinamo Moscow.
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If there is to be a player that will give Juventus particular trouble it may well be Claudinho, the Brazilian who has adapted swiftly to life in Russia. He was something of a thorn in Juventus' side in the first meeting between these two teams. His set pieces consistently found teammates as he created three chances on the night. It is worth noting here that Zenit are not the only side to pose Allegri's side issues at set pieces, only Sheriff Tiraspol have allowed opponents more shots off set pieces per game than Juve in this season's Champions League.
Claudinho also kept getting into shooting positions of his own. It took an impressive diving block from Leonardo Bonucci to deny his low drive that looked like it might otherwise have flown in at Wojciech Szczesny's far post. Against a Juventus side that can feel all too happy moving in a slow gear on the European stage, Claudinho can offer that burst of pace, energy and impudence that could earn Zenit the points they have merited in their meetings with Group H's big guns.
Atalanta vs. Manchester United: Spurs win prove to be false dawn
It looked excellent on paper. The away win wasn't just a 3-0 win drubbing of Tottenham but one in which they kept their opponent from registering a shot on target. After the shellackings their defense had received in recent weeks, this was the consummate defensive performance. Raphael Varane anchored a back three that kept Spurs from even testing David de Gea once. Certainly, there were things to like with and without the ball. As Ole Gunnar Solskjaer explained afterwards, there were numerous footballing reasons to make the switch away from a 4-2-3-1 to a 3-4-1-2.
"There's many, many things behind the tactics that we used today," said Solskjaer in his post match press conference. "One, of course, is solidity. We managed to get Raphael smack in the middle of that defense because we needed a clean sheet and we did. We got Edinson [Cavani] and Cristiano [Ronaldo] to play together as a front two. We got the width from the wing backs.
"Today we felt that worked and was going to work. If you change to a back three and don't get the result it will be scrutinized. The coaches have done a very, very good job to put that into place."
In the end though, it felt rather more like Solskjaer had come across an opponent at just the right moment. Nuno Espirito Santo was prepared to be even more conservative than Solskjaer was as he pursued of another few days in the job (they didn't come and now Antonio Conte seems on his way to Tottenham). United held firm but they did not ever look entirely at ease in their new system, particularly when they tried to use it in a more expressive fashion.
In the 51st minute Harry Maguire pushed up the left flank with the ball at his feet. Luke Shaw went with him. In such circumstances either a central midfielder ought to drop back or the right wing back Aaron Wan-Bissaka check his run. Neither do so and moments after Fred is caught in possession in the middle of the pitch Spurs find themselves one good pass away from a prime chance for Heung-min Son to equalize.
Only a failure of basic execution stood between them and a really good chance, one Giovanni Lo Celso somehow failed to pick out. There were several other moments in the match where Spurs brought about the same fate on themselves, failing to spot the sort of holes in the back three/five that are naturally going to pop up in a system that has had all of four days' work on it. Atalanta coach Gianpiero Gasperini might be bemoaning his side's final pass in a season where their vaunted attack has yet to hit top gear, but surely even they could complete that one above. It's a risky game relying on them to miss the same opportunities that Spurs did.
At the other end, United may have scored three goals but there were few periods in the game where they seemed to really exert pressure. Too much of the drive upfield came from Wan-Bissaka on the wing or Bruno Fernandes coming to get the ball and doing much of the offensive work individually. The visitors had 10 shots against Spurs but three of them were from the move that ended with Ronaldo's spectacular volley. The five attempts before that chaotic passage of play were worth a combined 0.24 xG combined and two of them were from more than 30 yards out. From the opening goal onwards United had two more shots, both of them coming as much from Spurs errors as their team's creativity.
In short, despite what the scoreboard said, this was no great attacking display. There did not seem to be much of a plan beyond relying on Ronaldo or Cavani to punish errors or provide moments of exceptional quality. They can do that frequently but the issue is that if they have an off day you have precious little to fall back on.
"I cannot give you the game plan and the tactics of course, but we have a squad to play many different ways," Solskjaer said in his pregame press conference. "You can put any tactics out or system out, it's the players within it and how they execute the role - that was the difference on Saturday.
"The quality of what we did was more important than the system we played."
Quality might trump system on occasion but there is a world in which both can be married to create something even better. It is hard to see that emerging from Solskjaer, who will find himself in reaction mode once more after losing Victor Lindelof to injury. That could mean bringing Luke Shaw inside, Eric Bailly being the next man up or ditching the back three entirely.
There do not seem to be any possible approaches that offer more than a temporary salve. Persevere with the plan that worked against Spurs and there will be precious little room for Marcus Rashford, Jadon Sancho and Paul Pogba as more than rotation pieces. Go back to the 4-2-3-1 that allows for more talent in the final third and you have to convince several forwards that defense is not something that happens without them. All of this because Lindelof, the third ranking center back, is injured.
This is the stage of the season where most managers at top clubs have settled on an approach that they can now tinker with and enhance. Manchester United seem to be firmly at the drawing board. It is hard to shake the sense that Atalanta will punish them for that.
Liverpool vs. Atletico Madrid: Klopp's side wobble
It is hard to find a stick to beat Liverpool with just now. They are an offensive juggernaut with 45 goals to their name from 15 games. All of Mohamed Salah, Sadio Mane and Roberto Firmino find themselves in some of the purplest patches of their career. Things aren't too bad at the other end too with Virgil van Dijk having swiftly settled back into anchoring the Reds backline, bringing the best out of full backs who can now bomb on without fear of the mess they're leaving behind them.
If there were a cause for concern it merely comes with what happens when Liverpool have unleashed enough of their attacking firepower to nearly win a game. Against AC Milan, Atletico Madrid and now Brighton in the Premier League they have looked to be firmly in the ascendancy only to let their opponents back into the match. One might even add Manchester City to that mix, a game where they led twice but ultimately drew, though such was the quality of both teams in that game that a draw seemed only reasonable. In Europe they got away with their lapses. Brighton were not so forgiving, turning a 2-0 deficit into a 2-2 draw at Anfield on "a day when [Liverpool} could have smashed them", according to Klopp.
"It looked like bit by bit by bit we lost another player, body-language wise," Klopp added. "Then all of a sudden it wasn't there any more." Perhaps Saturday's game will be the wake up call Liverpool need; next time they find themselves in such an advantageous position they will get the game won. Equally if any side can sniff out even the slightest drop in intensity and respond in ferocious fashion it is Atletico, who did so quite exceptionally at the Wanda Metropolitano last time out.
Antoine Griezmann may be missing for them this time but put Luis Suarez in his place and the intensity is unlikely to drop. It may just take the thud of a loss from a convincing position for Liverpool to kick their habit of frittering away strong positions. They should have the quality to see off Atleti but do not be surprised if they make it more difficult for themselves than need be.