The climax of the UEFA Nations League is here -- stream LIVE on ESPN+ (U.S. only) -- with some of the continent's biggest teams fighting for a place in the finals in October 2021, as well as preferable seeding ahead of the World Cup 2022 qualifying draw.

This year, preparations have been disrupted by COVID-19 -- due to both the virus and players battling an intense fixture pile-up, a number of Europe's best talents are unavailable. Finally, in this most turbulent of years, there is a growing unease around one of Europe's most successful nations, while others feel that this is the right time to bring in some exciting young talent.

Here's everything you need to know ahead of the final two rounds of Nations League group play.

- What is the UEFA Nations League?
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COVID chaos

The build-up to the Nations League has been a tangle of logistics, biosecure bubbles, COVID-19 testing and load management. But also muddying the waters are the differing quarantine rules from country to country, with the new restrictions on Denmark proving particularly taxing. The UK government has banned all travel from Denmark by non-UK citizens and has not offered an exemption for elite athletes. So, with Iceland playing Denmark on Sunday, Iceland's team, as things stand, will not be permitted to travel to the UK to face England at Wembley on Wednesday, leaving the two countries scrambling to find a neutral base for their game.

This is also affecting the Denmark squad with neither Tottenham's Pierre-Emile Hojbjerg, nor Brentford's Henrik Dalsgaard and Mathias Jensen, released for international duty.

COVID-19 has also put a direct dent in the Denmark squad with Hoffenheim's Robert Skov and a physio testing positive, while Belgium captain Eden Hazard has too and will miss out this week. Germany will be without Niklas Sule, Kai Havertz and Emre Can while striker Eden Dzeko will be absent for Bosnia and Herzegovina.

Then there is Italy. Manager Roberto Mancini, who has tested positive, named a bloated 41-man squad for their three matches -- one friendly against Estonia and Nations League games against Poland and Bosnia and Herzegovina -- given the upheaval at home due to the pandemic, that saw six Serie A clubs recently report positive results.

Injury toll

With a number of Europe's best players sidelined through injury, club managers will be keeping their fingers crossed that their stars return home unscathed after three bruising games in seven days. England are already sweating on Marcus Rashford's fitness after he injured his shoulder against Everton, Trent Alexander-Arnold will miss out following his calf injury against Manchester City on Sunday and Joe Gomez is facing an extended period on the sidelines after injuring his knee in training Wednesday.

Elsewhere, Spain are without Barcelona's Ansu Fati following a knee injury that's expected to keep him out for four months, and Germany star Joshua Kimmich is unavailable until January with a knee injury he picked up against Borussia Dortmund.

The Netherlands sent Steven Bergwijn back to Spurs after fitness tests, and Virgil van Dijk is a long-term absentee; Nathan Ake might be sidelined for weeks after suffering a muscle injury in Wednesday's 1-1 draw vs. Spain. And Belgium will be without Timothy Castagne, Leandro Trossard and Yannick Carrasco.

Portugal have Cristiano Ronaldo available despite the forward picking up an ankle knock at the weekend -- he scored once against Andorra in Wednesday's 7-0 romp after coming on as a half-time sub -- but the list of those on the sidelines is mounting up, much to the anger of their clubs.

Managers are absolutely fuming about the relentless fixture list. Manchester United manager Ole Gunnar Solskjaer called the Premier League schedule "an absolute shambles" and said the authorities had set his team up to "fail" ahead of their win at Everton on Saturday. Liverpool's Jurgen Klopp has previously been critical of the international break and the lack of rest time for the players, while Tottenham boss Jose Mourinho says when his players leave for national duty, "I'm never expecting good things, I'm only expecting negative things. Never good things."

Meanwhile, national team bosses have named larger squads not just to cope with any COVID-19 complications, but also to juggle their options. "It is important that we have enough players we can count on," was Bosnia and Herzegovina boss Dusan Bajevic's message after picking 32 players, but France manager Didier Deschamps has played down any concerns about the players hitting a wall.

"I wouldn't say there's any risk of burnout, but fitness is something that needs to be taken into account," Deschamps said. "These past weeks, the players played in nine matches: one every three days, including the European competitions. I can't say I'm not relieved when I see one of my internationals start on the bench for his club!"

Crunch matches for European giants

We are at the business end of the Nations League group stage, so pool matches are effectively "play-off games" for spots in the semifinals, and for promotion/relegation. Portugal-France in Group 3 on Saturday in Lisbon (2:45 p.m. ET on ESPN+) is a perfect example. Portugal are seeking to defend their Nations League crown and have named a squad of 25 players, with 37-year-old Pepe absent through injury.

"We can take the opportunity to look at players from our extended squad," Portugal boss Fernando Santos said, with one eye on the future. "[Potential debutants] Paulinho and Pedro Neto are part of that list of 40 or 50 players that I think are all very good. There are none of them that I do not trust or do not have quality."

On the other side of the ledger, France's form is very hard to gauge, especially after a shock 2-0 friendly defeat to Finland on Wednesday. Deschamps has tried various different formations -- 4-3-3, playing three at the back, a 4-4-2 diamond -- so you sense he's still testing the water for his best mix ahead of next summer's Euros. There are questions over Kylian Mbappe's fitness due to a hamstring injury, while Benjamin Pavard and Presnel Kimpembe are also doubts.

Deschamps has also spoken out about Paul Pogba's situation at Manchester United, saying "he cannot be happy, neither with his playing time, nor with his positioning."

This is also playing out against a backdrop of angst regarding the draw for the 2022 World Cup qualifiers. The top 10 European sides in the FIFA World Ranking will be seeded for the draw next month. Belgium, France, England, Portugal, Spain, and Croatia are all guaranteed Pot 1 seeds, but a bad week for Italy, Denmark, Germany, and the Netherlands, alongside a good week for Switzerland or Poland, could cause one of the more traditional European superpowers to face an awkward route to Qatar 2022 with only the group winners guaranteed to qualify.

Italy (Group 1) are on an unbeaten run of 20 games and Mancini was in bullish form ahead of round batch of matches, which started with a 4-0 friendly win over Estonia on Wednesday. "Poland are ahead of us in the standings? We'll win against Poland and we'll top our group," he said.

Denmark's preparations are up in the air due to COVID-19, and the Netherlands have made an average start to life under Frank de Boer. The Oranje are in real danger of missing out on a spot in the semifinals, drawing 1-1 with Italy last time out, while De Boer has also overseen a 0-0 draw with Bosnia and Herzegovina and a 1-0 defeat to Mexico. Their buildup has been disrupted by Bergwijn's unavailability, and Mohamed Ihattaren pulled out with an illness.

If Netherlands lose to Bosnia, and Poland beat Italy, then the meeting of the Dutch and the Poles next week is effectively a shootout for a World Cup qualifying draw seeding spot.

And all is not well with Germany (Group 4). There are dark clouds shrouding the national team, with general manager Oliver Bierhoff lashing out at the media earlier in the week, asking them to support the national team instead of being overly critical. He admits there is a growing gulf in relations between the country and national team, but while Bierhoff says he believes this is mostly down to the 2018 World Cup disaster, there's also a feeling the national side values financial spreadsheets and projections above everything else.

There is also a growing clamour for Germany to call up previously discarded veterans Thomas Muller, Jerome Boateng and Mats Hummels. They have been in outstanding form and Boateng and Hummels could solve Germany's problems in defence. Despite that, a comeback has been ruled out by manager Joachim Low and Bierhoff. If they slip up against Ukraine on Saturday, their place among the World Cup seeds could be in real doubt.

Germany's match against Spain on Tuesday (2:45 p.m. ET on ESPN+) is a shootout for a spot in the semifinals, and there are also plenty of questions around Luis Enrique's side. Spain suffered a poor loss to Ukraine last time out and there has been plenty of chopping and changing in the squad.

Who is Spain's No. 1 goalkeeper? Who is best placed to partner Sergio Ramos in defence? And who will score up front? It looks likely David De Gea will remain between the posts ahead of Kepa Arrizabalaga and Unai Simon, Pau Torres will play alongside Ramos, while they have called up a rejuvenated Alvaro Morata to answer their issues up front. "Since [Alvaro] Morata returned to Juventus, he is a different player both in attack and in defence," Luis Enrique said. Let's hope he manages to stay onside.

And in Group 2, favourites Belgium welcome England to Leuven on Sunday (2:45 p.m. ET on ESPN+) -- after the game was moved from Brussels due to the city's 10 p.m. curfew. England have to win the game to stay in contention for a place in the finals for the second successive edition. But in the back of the players' minds is the physical toll of this run of fixtures.

"It's all too much, not just for me," winger Thorgan Hazard said. "Champions League, Bundesliga, Nations League ... I don't want to complain too much because we have to do our job, but it is a lot.

"Well, here at the Red Devils, everyone will get playing time. The national coach has selected many players. Playing them in all three matches for 90 minutes would be difficult. The players will not burn themselves out. Our coaches also ask us to be careful, not to risk too much."

New faces

With all the absentees and upheaval, the international window gives managers the chance to introduce some youngsters. Borussia Monchengladbach's Marcus Thuram, 23, is looking to follow in his father Lilian's footsteps for France. He has been superb for Gladbach this season, with three goals and five assists in 11 matches, and is one of two uncapped players in Deschamps' squad -- 27-year-old Monaco defender Ruben Aguilar being the other.

Elsewhere, England could hand a debut to 17-year-old midfielder Jude Bellingham, called up following the withdrawal of Southampton's James Ward-Prowse. Bellingham joined Borussia Dortmund in the summer and has made 11 appearances to date, including 30 minutes off the bench against Bayern Munich at the weekend. If he makes his debut, the incredibly talented teenager will become the third-youngest England player in history after Theo Walcott and Wayne Rooney.

There are also potential debuts for exciting right-back Bote Baku for Germany, with uncapped defensive duo Philipp Max and Felix Uduokhai also named. Meanwhile, Belgium have selected Hertha Berlin striker Dodi Lukebakio in their squad.

Spain have Atletico Madrid's midfielder/forward Marcos Llorente in the party, while there is a recall for Arsenal's Hector Bellerin -- hoping to make his first appearance since 2016. Italy have a host of potential debutants with Alessandro Bastoni (Inter Milan), Davide Calabria (AC Milan), Gian Marco Ferrari (Sassuolo), Luca Pellegrini (Genoa), Matteo Pessina (Atalanta), Mattia Zaccagni (Hellas Verona) and Pietro Pellegri (Monaco) all in the squad.

Note: Additional reporting by Stephan Uersfeld, Julien Laurens, Dale Johnson, Alex Kirkland, Sam Marsden and Andrew Cesare Richardson