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The first legs of the round of 16 conclude this week with some of the most intriguing fixtures on the European calendar so far this season. Tuesday pits the two defining sides of Diego Simeone's European career against each other as he returns to Inter with his Atletico Madrid side. Wednesday then brings a battle of the faltering champions as Napoli and Barcelona face off. All that and Arsenal returning to the knockout stages while PSV Eindhoven try to translate their stunning domestic form onto the European stage. Here are four players to look out for:

1. Marcus Thuram, Inter

This has been the season of Lautaro Martinez, the Argentine timing his leap in excellence to coincide with that of Inter, who return to Champions League action looking to be even more intriguing dark horses than 12 months ago when they made it all the way to the final. Martinez is their X factor, the sort of forward who could swing a tie in his side's favor, but that status can weigh heavily on even the greatest. Lucky for the Argentine then that he has a running mate more than capable of easing the burden.

Indeed if it weren't for Martinez Thuram might be the name on everyone's lips. The France international, picked up on a free transfer from Borussia Monchengladbach last summer, has 12 goals and eight assists across 34 games in all competitions this season, solid numbers that still don't tell the full story of his form in an Inter shirt. Thuram is a selfless, energetic forward who is more than happy to chase lost causes and move into the channel, taking a center back with him to forge space for Martinez. Take this assist, one of four he has laid on for his senior strike partner in Serie A this season.

Thuram leads a break down the right before crossing for Martinez Wyscout/Serie A

Even before the pass is played into the right channel, Thuram is sprinting forward to attack that space. His pace and one-on-one trickery takes him away from Bremer, one of Serie A's best defenders, before he times a delivery for the exact moment that Martinez has cut across his defender. That is the sort of understanding that usually takes years for a strike force to develop; Martinez and Thuram have done so on fast forward.

Increasingly the new signing is doing more than merely serve the main forward. He opened the scoring in the 4-0 rout of Salernitana at the weekend, a goal on the back of an even more impressive display when Inter quelled the Stadio Olimpico to beat in-form Roma. His smart run across the center back got his side their equaliser in the second half and it was then his pressure that forced Angelino to turn into the net for Inter's decisive third goal. "I always try to either get to the ball, or else put pressure on the defender," he said after, comments that typify Thuram: an almighty headache for defenders with more than enough excellence to punish their mistakes. The trouble for Atletico is he isn't even the hardest assignment on their scouting report.

2. Luuk De Jong, PSV Eindhoven

Trouble's brewing in Eindhoven. Two draws, yes, two! There will be no perfect season for the Eredivisie leaders, a side whose domestic record now reads at an embarrassing played 22, won 20, draw two, lost none, scored 70, conceded 10. Will Peter Bosz be able to pick his players up off the mat now they have dropped points twice in their last five games?

Whether one is being serious or not, it tends to be an almighty challenge to assess any Dutch representatives at this stage of the competition. For every side that captures the imagination of Europe on a deep run (Ajax, 2018-19) there is another that starts brightly but fritters things away with a defeat to relatively ordinary opposition (Ajax, 2021-22). It is even harder to get a grasp on which side PSV might be given that they came through one of the most favorable of group stages; finishing ahead of Spain's 15th best team and France's sixth hardly screams Champions League dark horses.

If any player typifies the challenge of understanding PSV it is Luuk De Jong, the 33-year-old who has underwhelmed almost everywhere he has been outside the Netherlands (though he did score some big goals in the Europa League for Sevilla). In 22 Eredivisie games this season, he has 19 goals and eight assists. Defenses have no answer for this penalty box predator, one who doesn't just win all the aerial duels you'd expect of a 6-foor-1 striker but who also does an impressive trade in first-time finishes off his left foot.

The same was true in qualifying for the Champions League, where he put Sturm Graz and Rangers to the sword. Then came the group stages, where De Jong was restricted to just 15 shots, more than half of which came in the desperate but ultimately successful scrabble for a draw against Sevilla. His 447 minutes in the competition brought two goals, one a penalty, and an average of 0.23 non-penalty expected goals (npxG) per 90 minutes. That sort of return will not be good enough against Borussia Dortmund, who might not be setting the Bundesliga alight this season but have not conceded two goals in a game since early December.

Non-penalty shots taken by De Jong in the Champions League group stage, sized by xG value TruMedia

If the Germans' backline does not feel a threat from the center forward position then they can shade towards the real danger men of this PSV side: the fearless, shoot-first ask questions, such as, 'Should I actually have passed there?' later wing tandem of Noa Lang and Johan Bakayoko. For those two forwards to be the best version of themselves, they need an option in the penalty area, a center forward who can test Mats Hummels et al. Can he be Eredivisie De Jong in the Champions League? If so, PSV have a chance of making some noise in this round.

3. Gabriel, Arsenal

If Arsenal are going to win the Champions League -- and there are eminently plausible scenarios in which they do -- then it will be off the back of their defense, one which right now might be the best in Europe. Over the last five Premier League games, they have given up shots worth a combined 1.72 xG. By way of comparison, Manchester United allowed Luton Town to get up 1.74 xG on Sunday afternoon. These are Thomas Tuchel at Chelsea numbers. Champions League titles are built on elite defenses. That is what Arsenal have.

Touches by Arsenal's opponents in the attacking half during the last five Premier League games TruMedia

Much of the credit for that goes to Declan Rice, the £105 million signing who has become essential to his new side's ability to kill off counters at source, and the world's best young center back, William Saliba. When your defense is as good as Arsenal's, however, there is plenty to go around and the spotlight deserves to shine a little bit brighter on Gabriel. Usually, this would lead to a celebration of the almighty threat he poses from set pieces. No one in the Premier League has scored more off them than the 14 he has since he arrived in the summer of 2020.

Even framing him as an attacking weapon rather does down the Brazil international, who has morphed into nothing less than a top-tier defender over his near-four years in England. Generally asked to cover the left flank when his fullback inverts, Gabriel has proven himself to be a monster in duels, ready to step up and assert his will on opposing forwards. He does so in the knowledge that if they do spin him he is more than capable of recovering in behind. At 26, he is winning more aerial duels than ever before, but that aside, he has that most encouraging of statistical trends for defenders at top sides: he is doing less and less of everything. Fewer clearances, fewer duels: some of that comes down to Arsenal's own control of games but so much is also down to Gabriel controlling defensive encounters so well that he does not have to do the stuff that shows up on the stats sheet.

When Gabriel first arrived at Arsenal, the talent was apparent but his exuberance could on occasion be his downfall. Here was a player who knew he could play the pass or carry the ball upfield. Now he is sufficiently confident in his standing at the club that he does not feel the need to show off that talent. The sloppy errors that pockmarked his early days are all but eradicated: not since August 2022 has he made what Opta class as an error leading to a goal. It was in October of that year that he made his last error leading to a shot.

No wonder he is now entrusted with the central center back role in Arsenal's rest defense, a job that had previously required the combination of explosive pace and cool-headedness that Saliba defines. Gabriel's growth gives Arsenal versatility, they can now have their left back tuck in to form the back three in the way that Ben White usually has. That inversion from the opposite flank drags away support defenders who might follow an overlapping White to deal with Bukayo Saka, now they are sucked into the midfield. All this in part because Gabriel has proven he can now anchor the Arteta system.

He and his teammates are eminently capable of putting in 180 minutes of blemishless defense. If they do so then Porto will have an almighty task ahead of them just scoring, let alone staying in this tie.

4. Lamine Yamal, Barcelona

It's Lamine Yamal's classmates I feel sorry for. While they're hunting for fake IDs and worrying about exam grades, they're seeing the tribulations of an average teen life put into the shade by a youngster carrying the offense at one of the biggest clubs in world football. In the last 10 days alone, Yamal has scored a brace to earn Barcelona a draw against struggling Granada before providing an assist and winning a penalty for Robert Lewandowski as the side snuck past Celta Vigo. With his 17th birthday not until July, Yamal has over 22 hours of La Liga football to his name this season, something which should prompt significant questions in Catalonia over the long-term use of a youngster who is still growing into his body.

Equally, you can see why Xavi so frequently turns to his teenage flier. Yamal is delivering performances that would be impressive from a player 10 years his senior. According to fbref, his combined npxG and expected goals assisted of 0.61 places him in the 91st percentile of players across Europe's top five leagues. A fearless ball carrier who does more than his fair share of defending, he would probably be just as integral to Barcelona even if the likes of Gavi, Ferran Torres and possibly Joao Felix weren't sidelined for the trip to Naples.

How Lamine Yamal compares to other La Liga forwards in the 2023-24 season TruMedia

Whatever the age, Yamal has the look of a player who could cause all sorts of problems for a left flank that was not particularly strong for Napoli last season in defense and looks all the weaker this. Whether it's Pasquale Mazzocchi or Mario Rui lining up, whether in a back three or a five, they are going to be in for an almighty challenge when Yamal runs at them.