The X-factor for every team left in the Champions League

The Champions League knockout stage kicks off this week with a number of great matchups, and each of the 16 remaining teams will rely heavily on their big stars to try and get them to the final. So often, though, it’s a less heralded player that proves to be the difference between an early exit and a deep tournament run.

These are each team's x-factors:

Giroud lost his starting No. 9 spot to Alexis Sanchez this season, but since the new year it’s been the much-maligned Frenchman – we’ll never agree on whether he’s underrated or overrated – who’s come up big when it mattered most. Giroud’s late goals have saved Arsenal points multiple times already in 2017, and if the Gunners are finally going to get past Bayern in Europe, they may need more of his heroics.

The good news: Giroud has three goals against Manuel Neuer in five games, more than all of his Arsenal teammates combined.

Antoine Griezman remains Atletico’s undisputed star, Koke its brains, and the defense its bedrock, but as for their X-factor, look no further than Carrasco. The 23-year-old Belgium star announced himself to the world by coming off the bench to send last year’s Champions League final into extra time, and he’s built on that with 11 goals (second-most on the team) and two assists in all competitions this season. Carrasco has been the ace up Diego Simeone’s sleeve, and he’s primed for a bigger role in the knockout stage.

Luis Enrique recently dubbed him “Harry Potter,” and the nickname fits. While Barcelona’s front three remains the most formidable in the world, “M-S-N” produces considerably less magic without Iniesta pulling the strings behind them. When Iniesta missed all of November with a knee injury, Barcelona only managed to win three of their six games, and his absence was particularly glaring in a 3-1 loss at Manchester City. But when he’s been healthy, Iniesta has been his usual calm, reliable, dominant self. At 32, Iniesta might still be the finest midfielder in the game, and he remains Barca’s most indispensable player not named Messi.

As if Atletico Madrid wasn’t a difficult enough draw, now Leverkusen have to do without playmaker Hakan Calhanoglu, who was handed a four-month ban by FIFA last month. Leverkusen will need everyone to pick up the slack, but Brandt is the guy to watch. Name any big club in Europe, and chances are they’re monitoring the 20-year-old rising star. Brandt loves to take on defenders, leads the team with six Bundesliga assists this season, and he’s a good finisher as well. Look for him to test Atletico’s vaunted defense.

Manuel Neuer and Robert Lewandowski are the only irreplaceable stars on this deep roster, but Bayern also need Thiago at his best to reach their first Champions League final in four years. Though he’s primarily used as a holding midfielder in Carlo Ancelotti’s flexible 4-2-3-1, Thiago might also be Bayern’s best option as a true No. 10 with Thomas Mueller struggling so much this season. It’s a role he dominated in a 3-0 win over RB Leipzig in December, and given the team’s wealth of options in defensive midfield, one he could find himself in again vs. Arsenal. Regardless where he lines up, if Bayern’s metronome is able to dictate the flow of the game, this team is pretty much unstoppable.

Toto Salvio and Rafa Silva might be the bigger stars, but no Benfica player has been better than Pizzi this season. The 27-year-old is a bit of a late bloomer, only really breaking into Benfica’s first team last season, but he’s quietly turned into the club’s key man in midfield and top scorer, notching 11 goals in all competitions. None of those have come in the Champions League, however, where Pizzi has struggled to make as much of an impact. If Benfica want to spring an upset against Dortmund, Pizzi will need to be on his A game.

What more can be said about the 19-year-old who’s taken the Bundesliga by storm? Dembele has speed and trickery for days, leads the league in assists and successful dribbles per game, and has been relied on more and more by Thomas Tuchel. Though the Frenchman still has lots of room to grow, especially tactically and in terms of his decision-making, Dembele’s impact on games over the right wing has been undeniable. Can he continue his ascent on the biggest of stages over the next few weeks and, potentially, months?

Gonzalo Higuain and Miralem Pjanic were Juve’s marquee summer signings to offset Paul Pogba’s departure, and they’ve been excellent, but Dybala is the team’s pivotal difference maker. The 23-year-old Argentine, who’s drawn (unfair) comparisons to Lionel Messi, is Serie A’s – if not all of Europe’s - brightest rising star, and the one his teammates look for to create something out of nothing. Despite a slow start and missing nine games earlier this season, the 23-year-old has eight goals and four assists to his credit after a stellar 23-goal, nine-assist debut campaign with the Bianconeri.

It’s simple really. Last season Vardy scored 24 goals as Leicester clinched the most improbable, shocking Premier League title ever. This season, Vardy has just five goals – none in the Champions League – and the Foxes are in serious relegation danger. Leicester have plenty of other problems, but the anemic attack is their biggest right now. Incidentally, Vardy’s only great performance this season had led to Leicester’s best win, when he scored a hat trick against Man City. Leicester need Vardy to throw another party if they don’t want their debut Champions League campaign to end at the hands of Sevilla.

Three months ago this would have been a bad joke, but City’s hopes in Europe might well rest on the shoulders of the one guy most had already written off at the Etihad in the fall. Toure wasn’t even included in City’s squad for the group stage, but since yet another season-ending injury to Ilkay Gundogan in November, Pep Guardiola’s had no choice but to turn to the club’s longtime linchpin to run the show in midfield again. The results have been mixed. Toure’s been exceptional at times and exceptionally bad in some of City’s recent losses. But with only the equally inconsistent Fernandinho as an alternative, City will have to ride or die with their 33-year-old club legend.

Monaco boast the top-scoring offense in Europe’s top 5 leagues this season and it’s not particularly close. The rejuvenated Radamel Falcao has played a big role, but it’s Monaco’s slew of young talents that have usurped PSG as the new Ligue 1 favorites, and the 21-year-old Lemar is the best of the bunch. Only Falcao has scored more goals than the lightning-quick winger this season and only Bernardo Silva has tallied more assists. With Pep Guardiola’s struggling Man City side ripe for an upset, Lemar could be the catalyst to make it happen, especially given City’s fullback issues.

Even after losing Gonzalo Higuain to Juventus last summer and seeing his replacement, Arkadiusz Milik, go down with a torn ACL in the fall, Napoli have still managed to score 10 more goals than any other side in Serie A. That high-flying offense will test Real Madrid as well, and while most eyes will be on captain Marek Hamsik and Dries Mertens (20 goals in all competitions), Insigne could end up stealing the show. With seven goals and five assists to his name this season, Insigne has been one of Napoli’s core contributors, and though he hasn’t scored a Champions League goal in over three years, the Italy striker could produce something special at any moment.

Draxler was absolutely dreadful for Wolfsburg this season, but he’s been phenomenal since his January move to Paris. It's only been eight games, but Draxler already has four more goals for PSG than he scored for the Wolves this season (it was zero). The Germany winger is finally showing the dynamism again that had made him one of the most desirable transfer targets of the past couple years. Draxler has already displaced Angel Di Maria in Unai Emery’s 4-3-3, and with Round of 16 opponent Barcelona most vulnerable over the wings, PSG’s best bet at an upset might lie with their new star.

All eyes will probably be on the uber-talented Andre Silva and midfield linchpin Danilo Pereira. Porto’s three-headed Mexico contingent of captain Hector Herrera, ‘Tecatito’ Corona and Miguel Layun will draw some attention as well. But it’s ol’ Saint Iker who could be the key to a surprise run in the knockout stage. On this youthful Porto team that’s loaded with talent but scant on experience, Casillas’ leadership and mindboggling track record of winning – is invaluable. It doesn’t hurt that Casillas has been in stunning form between the posts. The Real Madrid legend gave up just three goals in the group stage, anchors the best defense in Portugal, and is making ridiculous saves like this. Not bad for a 35–year-old.

You’d never think of him first, second, third or even fourth, but Casemiro might be Real’s most important player. With Cristiano Ronaldo, Gareth Bale and Karim Benzema wrecking havoc in front of the technically brilliant but defensively challenged Toni Kroos and Luka Modric, Casmerio has been counted on alone to provide cover and sniff out opposing counterattacks. When the Brazilian defensive midfielder went down with a broken fibula in September, Real struggled for quite some time. They can’t afford to lose him again.

The surprise team of La Liga is known for its high-octane offense, so why are we going with a defender? For one, Sevilla’s direct, fluid style is born out of the back, with Pareja & Co. being relid on to launch attacks quickly. And then there’s the fact Sevilla simply can’t seem to win without their vice captain on the field. The Palanganas have just three wins in 11 games without Pareja this season, and two of those came against a fourth-division side in the Copa del Rey. Samir Nasri and Sevilla’s attackers might be getting most of the glory this season, but they couldn’t do it without their big center back.