The UEFA Champions League returns this week with the round of 16, with four first legs happening this week and the remaining four first legs scheduled for the week of Feb. 22. The pick of this week's ties is undoubtedly happening at the Camp Nou on Tuesday, with Barcelona hosting Paris Saint-Germain in what promises to be a blockbuster clash.

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Will Lionel Messi spark the Spanish giants to a comprehensive win? Can Barcelona avoid another European disappointment? As for PSG, how will they fare without Neymar, whose latest injury rules him out from yet another big Champions League clash? Will Kylian Mbappe lead them to victory instead? And how will the two managers, Ronald Koeman and Mauricio Pochettino, handle their respective squads?

ESPN's Sid Lowe (Barcelona) and Julien Laurens (PSG) report on the mood around each team as they prepare for what could be a pivotal game in the two sides' respective seasons.

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Barcelona: Not the pushovers in 2021 that they were in 2020

There's something relentlessly chirpy about Riqui Puig, so it wasn't much of a surprise to see him standing there on Saturday night, wearing a smile the size of the stadium. After all, Puig is the kid who described himself as "a happy lad" and refused to complain when he wasn't playing, insisting: "I have my family, I have my health." And yet this was different, deeper; this time there were more reasons to be cheerful, and not just for him.

Puig had just completed 90 minutes for the first time, Barcelona had put five goals past Alaves and next up, they face Paris Saint-Germain in the Champions League on Tuesday night believing that they could even win. "We're all really looking forward to it; we can't wait for the day to come," Puig said, all bright-eyed and bushy haired, teeth nice and clean. "And I think we're going to measure up."

Not so long ago, you might have laughed; now Riqui may just be right. And the fact that there's even a chance he is right is quite something.

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When the draw for the last 16 was made, Barcelona were out. That, at least, is what most people thought would happen. And although none of them would admit it, still less publicly, those people probably included their own players. It certainly included most of their fans. Defeat in the final group game, 3-0 at home to Juventus, weighed heavy, revealing their problems and leaving them in second place, exposed to an opponent like this and with little hope of progressing.

At that point in the season, Barcelona had been beaten by Atletico and by Cadiz, their fourth defeat in just 10 league games. "Broken," the headline in El Mundo Deportivo had run after the Atlético game; "it's a gigantic step backwards," Ronald Koeman had admitted after the Cadiz game. Even more humiliating, he heard Cadiz's coach talk openly about just how easy it had been to defeat his team, which found themselves ninth, closer to the relegation zone than the Champions League places.

Ten games in, no Barcelona team had been worse since 1971. Both Sport and Mundo Deportivo were selling Barcelona face masks for only €9.99, which wasn't even cheap. "Get your best defence," the slogan ran, which was an open goal for amateur comedians everywhere, just as the way they were playing was an open invitation for opponents everywhere.

Those defeats were followed, the day before the Champions League draw, by a 1-0 win over Levante. But if at least they had won, the final image of the game summed it up: Marc-Andre ter Stegen kissed the ball in thanks, relieved that the final shot had ended in his hands, relieved to have survived. They then drew two of their next three games before scraping through 1-0 against bottom-of-the-table Huesca, one newspaper bidding goodbye and good riddance to 2020, that "Damned Year."

But here's the thing: 2021 has been different. They've won five in a row in La Liga, despite playing four of them away. They won three in a row in the Copa del Rey, all of them away from home. They beat Real Sociedad in the Super Cup semifinal, on penalties. They played 10 games and lost only one -- and that was in extra time in the Spanish Super Cup final. It hurt, and hurt a lot, but it can happen.

Something was shifting, settling. The pieces seemed to be falling into place.

Messi actually looked ... happy. In Pedri, he'd found someone to enjoy playing with. He seemed to be embracing a new role, a different generation around him, and perhaps was surprised at how good these guys might be. Frenkie De Jong was suddenly everywhere in midfield. Ronald Araujo emerged, their centre-back for the next 10 years, according to incumbent Gerard Pique.

The Jordi Alba conundrum remains, but the balance is tilted to "Good Jordi" right now. Sergio Busquets has started to take control again in his holding midfield role. Ousmane Dembele stayed fit, finally getting continuity on the right, giving Barcelona the ability to stretch teams and boasting an acceleration that perhaps no one else has, anywhere. "He's happy, he's fitter," Koeman said. Even Antoine Griezmann seemed to have found himself, the feeling that he mattered: It wasn't the goals, it was a role.

Slowly, it actually started to make sense. The structure shifted -- Koeman revealed himself to be pragmatic and more flexible than he'd first seemed -- and it all felt more rational. That huge hole in the middle was no longer there, the team not so split: There was an actual formation into which players slotted. That 4-2-3-1 (which was more like a 4-1-chaos) became something a little more like a 4-3-3, if not entirely. Pedri, De Jong and Griezmann were among the best players in Spain in January. And Messi, of course. Koeman even insisted that his players deserved a 10/10.

It wasn't that good and it wasn't perfect, far from it. They still had injuries: Pique, Ansu Fati, Sergino Dest, Philippe Coutinho, Sergi Roberto. They still made mistakes, too, and had a sense of vulnerability about them, although they were more individual than systemic now (Samuel Umtiti in particular). Those cup wins had all required extra time, against Cornella, Rayo Vallecano and Granada. They were hardly imperious. And of those periods of extra time, added to the Real Sociedad and Athletic Bilbao games in the Super Cup, made 150 minutes more in a period where they were playing every three days anyway, the tiredness taking hold.

The impact of that could be yet to reveal itself. From Dec. 28 to the PSG game Tuesday, they will have played 13 matches plus five extra times, the equivalent of a game every three days for 49 days, and only two of those at the Camp Nou. "It's a difficult moment; lots of extra time, travelling, getting home late ..." Koeman admitted.

That's not all. They still rely on ter Stegen at times -- too many times. You wondered, too, what would happen when they played a really good team: They, after all, had beaten Betis, Elche, Granada, Rayo, Athletic, Cornella and Real Sociedad (on penalties). Perhaps part of the answer came last week when they faced Sevilla at the Sanchez Pizjuan and were beaten 2-0, though Koeman claimed that they had played well and deserved more.

While he would say that, amid it all something was building, emerging. A team, perhaps. The late, dramatic comeback against Granada suggested so. Barcelona were better, that's for sure. There was something there now that simply hadn't been there before. Some hope, for a start.

And while Sevilla was a setback, next came Alaves and five more goals. Francisco Trincao, the last to come on board, got two of them. Messi was Messi, and there is nothing more a player can aspire to. It had been the kind of win that allows you to dream of defeating PSG, Marca said. "A test for PSG," one Catalan headline called it. If so, they had passed.

By Tuesday, they hope to have Araujo and Dest back and healthy. On Sunday, even Pique was making one last push to be available. Suddenly there was hope. From no chance, there was a little light. Puig was smiling. Koeman, too. "We're in good shape, and the team is confident," he said. "This is a good moment for us, we've been playing well for six or seven games. It will be even."

He might even be right. And that's not something even the most optimistic Barcelona fan would have said when the draw was made. Well, Puig might, but no one else. -- Sid Lowe

Paris Saint-Germain: All eyes on Mbappe

The curse has hit again. For the third time in his four seasons at PSG, Neymar is injured in February and will miss yet another huge European game for the club. He won't be on the pitch on Tuesday night when the French champions take on his former team. It is a huge blow, of course, and one made worse by the fact that Angel Di Maria is also out. Lately, the pair have been PSG's best players.

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Neymar on the sideline is far too common, as the 29-year-old has been absent or unable to play roughly half the games since he arrived in the French capital in 2017. It is a staggering statistic, though, of course, no one inside the club is blaming Neymar for it. They have rallied around him once again. No one is blaming Mauricio Pochettino, either, for playing his essential No. 10 in a pointless French Cup match against a second-division club in the freezing cold and on an uneven pitch. "Ney" needed game time after not playing much so far in 2021. Yes, he got kicked at Caen on Wednesday; he always does. Is it the reason he's out? No, although it didn't help.

Around the club, they're understandably staying positive. This is still a very talented squad with the potential to square up with Barcelona. Liverpool beat the Spanish giants 4-0 two seasons ago with Xherdan Shaqiri and Divock Origi, not Roberto Firmino and Mohamed Salah, in their starting line-up. There is no panic or fear at PSG about facing this Barcelona. Even without Neymar and Di Maria, PSG are confident they can go through. Pochettino just has to find the right formula.

So far, Pochettino's arrival has been positive, but the quality of football is not great yet. And to play well and win, he will have to rely on Kylian Mbappe.

Since his incredible Champions League breakthrough season with Monaco in 2016-17, when Mbappe took Europe by storm and scored in five of six knockout games at only 18 years of age, the French prodigy has not had it all his way in the top club competition. He had to wait until December to score a Champions League goal in 2021. He failed to lead his team to the quarterfinals in 2019, despite two big chances against Manchester United in the last 16 second leg, at the Parc des Princes, in Neymar's absence. He was anonymous against Real Madrid in the last 16 two legs in 2018 in Neymar's absence.

This time, Mbappe has to deliver. On Tuesday, he will have to dominate at Camp Nou and show he can take the leadership mantle from Neymar when his teammate is unavailable. This doesn't mean he needs to dribble past everyone and try to play like Neymar; it means making the right decisions, being efficient, playing with maturity and letting his talent do the talking. Mbappe is still undecided about his future at the club -- whether to extend his current deal or leave -- unlike Neymar, who will sign his new contract soon. While PSG are content to wait while Mbappe makes his mind up, he could be especially inspired to guide them towards a key victory on Tuesday.

Of course, Mbappe can't do it all on his own. He will need Mauro Icardi to also be efficient in front of goal against a club he knows well after his three-year development at Barca's La Masia. Marco Verratti will need to dictate the game in midfield against the club he almost joined a few years ago, too, while Marquinhos, Presnel Kimpembe and Keylor Navas will need to be at their best defensively and deny Messi & Co. around goal.

Ah, Messi. The Argentine has been at the centre of all the build-up to the Champions League last 16. Barcelona have not been happy that one of the clubs he's been linked with, PSG, have publicly declared their desire to sign him in the summer. Yet PSG don't really care about what Koeman or people at Barca say about it. They want to believe that they have a chance to sign him in the summer if he decides to leave. A win in this two-legged tie would certainly make the case for Paris. -- Julien Laurens