The Champions League is back, and back with a bang, after the draw for the 2020-21 group stage concluded on Thursday in Geneva. The top line? A head-to-head between Cristiano Ronaldo and Lionel Messi as Juventus and Barcelona were paired together in Group G, in a game that could also see fans inside the stadium after UEFA confirmed grounds can be up to 30 percent full for fixtures if local regulations permit. The stand-out pairing of the group stage also boasts an American sub-plot with U.S. teammates Weston McKennie and Sergino Dest facing off against each other.

Champions League draw: Messi vs. Ronaldo as Barca, Juve meet

With Italian champions Juve and La Liga runners-up Barcelona having not met competitively since November 2017, the Group G clashes will see Ronaldo and Messi renew their on-field rivalry for the first time since the Portuguese forward left Real Madrid for Juventus in 2018.

Ronaldo and Messi have played against each on 33 occasions in competitive club games, and Barcelona's Messi has so far come out on top 15 times, with Ronaldo winning on nine occasions at past clubs Real and Manchester United. The two men have dominated the Champions League, with Ronaldo winning five European Cups -- two more than Messi. Ronaldo also tops the all-time scoring charts with 130, 15 more than Messi, his closest rival.

But while the match-up between football's two biggest stars is the headline from the Juventus v. Barcelona pairing, the game could also see two of the USMNT's brightest talents take to the stage. Juventus midfielder McKennie completed a summer move to Turin from Schalke, while defender Dest has just sealed a permanent move to Barcelona from Ajax.

Done deal! USMNT's Dest joins Barcelona

Away from Group G, reigning champions Bayern Munich have been placed in Group A against Atletico Madrid, FC Salzburg and Lokomotiv Moscow. And Group H promises to be the most unpredictable, with last season's runners-up Paris Saint-Germain paired with Manchester United, RB Leipzig and Istanbul Basaksehir.

So with the group stages due to start on Oct 20-21, ESPN has assessed how they will shake down and which teams will make it through to the Round of 16.

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GROUP A: Bayern Munich, Atletico Madrid, RB Salzburg, Lokomotiv Moscow

The German and Champions League winners, Bayern, will be strong favourites to make it to the knock-out stages, with Atletico Madrid and FC Salzburg likely to be battling it out for second spot in the group.

Salzburg have lost Erling Haaland and Takumi Minamino since almost advancing beyond last season's group stage, but the Austrian champions have proven to be masters of recruitment and rebuilding. Atletico should not underestimate the threat they will pose, even with their new strike pairing of Diego Costa and Luis Suarez.

Lokomotiv Moscow, who finished runners-up to Zenit Saint Petersburg in Russia's Premier League, finished bottom of their group, winning just once, in last season's competition and they're facing the same fate this time around.

TO ADVANCE: Bayern Munich, Atletico Madrid

MUST-SEE MATCH: The two games between Bayern and Atletico won't just represent the best of the group, but will also give a genuine look at how well they're constructed for the presumptive pressure of the knockout rounds. Plus it'll be fun to see Luis Suarez, now in Diego Simeone's first XI, and Joao Felix trading punches with Bayern's fearsome forward corps. (Oh, and they no longer have wily veterans like Diego Godin to hold down Atletico's rearguard.)

GROUP B: Real Madrid, Shakhtar Donetsk, Inter Milan, Borussia Monchengladbach

On paper, Real Madrid appear to be the favourites to qualify, but it is an evenly-balanced group, with each team capable of beating each other.

Zinedine Zidane's Real should top the group, but Internazionale are an emerging force again under Antonio Conte, with the goals of Romelu Lukaku firing the team to last season's Europa League final. Ukrainian champions Shakhtar are seasoned performers in the Champions League and a dangerous opponent, particularly in Ukraine, while Monchengladbach have become one of the leading teams in Germany under coach Marco Rose.

TO ADVANCE: Real Madrid, Inter

MUST-SEE MATCH: Inter Milan vs. Borussia Monchengladbach might not sound like much, but it'll be a tactical battle and, most likely, the games that determine who joins Real in the round of 16. Conte and Inter have enough talent, front to back, to advance, but Gladbach's attacking flow will unsettle all comers in Europe as it does in Germany.

GROUP C: FC Porto, Manchester City, Olympiakos, Marseille

Portuguese champions FC Porto are the seeded team in Group C, but the reality is that Manchester City will comfortably be the side to beat in this section. Pep Guardiola's team, who were surprisingly eliminated by Lyon in the quarterfinals in Lisbon last season, should coast through this group.

Marseille, coached by former Porto, Chelsea and Tottenham manager Andre Villas-Boas, will back their chances to make it out of the group, especially if they able to recreate the typically-hostile atmosphere inside Stade Velodrome with a limited number of supporters inside the ground.

TO ADVANCE: Man City, Marseille

MUST-SEE MATCH: Porto vs. Marseille will be the fight for second place but also a chance for Villas-Boas, who won the Europa League with Porto in 2010-11, to show how far he's come in his career after two humbling gigs in the Premier League.

GROUP D: Liverpool, Ajax, Atalanta BC, FC Midtjylland

Liverpool should make comfortable progress through the group. Ajax and Atalanta, the surprise packages of the last two Champions League campaigns, could make life difficult for Jurgen Klopp's team at home, but even without the backing of a noisy and passionate Anfield, Liverpool will be too strong for their rivals.

As for Midtjylland, they will be hoping to cause an upset by finishing third and qualifying for the Europa League in their first Champions League group stage appearance.

TO ADVANCE: Liverpool, Atalanta

MUST-SEE MATCH: With all due respect to Ajax, seeing how Liverpool and Atalanta's all-out attacking forces matches up will be the thing that thrills in Group D. Both sides boast a fearsome array of goalscoring weapons and both sides like to force their imprints on any opponent. Will either side dare back down?

GROUP E: Sevilla, Chelsea, Krasnodar, Rennes

Europa League winners Sevilla and Chelsea will expect to battle it out for the top two positions in what appears to be the most straightforward group to emerge from the draw. This is a group of extremes, as two clubs with serious European pedigree will take on two others -- Stade Rennais and Krasnodar -- making their first appearance in the Champions League group stages.

Both Stade Rennais and Krasnodar sealed qualification for the competition by finishing third in their domestic leagues, so Sevilla coach Unai Emery and Chelsea manager Frank Lampard will quietly believe that they have landed a dream draw.

TO ADVANCE: Sevilla, Chelsea

MUST-SEE MATCH: Sevilla's European bonafides are hard to dispute, but this will be the time when Chelsea expect their new signings, like Timo Werner and Kai Havertz, to show their class given a wealth of their own experience in this competition. Tune in and watch the fun.

GROUP F: Zenit St. Petersburg, Borussia Dortmund, Lazio, Club Brugge

Borussia Dortmund will be favourites to top this group, especially if Manchester United target Jadon Sancho remains at the club to build on his forward partnership with Erling Haaland.

Simone Inzaghi's Lazio will be hopeful of making it to the Round of 16 too, with Russian champions Zenit Saint Petersburg often struggling to repeat the domestic form in the Champions League. Second spot is likely to be a two-way battle between Lazio and Zenit, with Club Brugge making up the numbers.

TO ADVANCE: Dortmund, Lazio

GROUP G: Juventus, Barcelona, Dynamo Kyiv, Ferencvaros

All eyes will be on the two matchdays that see Ronaldo and Messi face off, because this really is a group of two heavyweights and two minnows, in Dynamo Kiev and Ferencvaros.

Both Kiev and Hungarian champions Ferencvaros have proud histories in European competition, but in terms of the modern era, they're likely to be fighting for third spot and a place in the Europa League while the big two slug it out for top spot. Barcelona's 8-2 meltdown against Bayern Munich in last season's quarterfinals highlighted the problems at Camp Nou, so Juventus will be expected to shade the head-to-head games.

TO ADVANCE: Juventus, Barcelona

MUST-SEE MATCH: Juventus vs. Barcelona. Need we say more?

GROUP H: Paris Saint-Germain, Manchester United, RB Leipzig, Istanbul Basaksehir

One big club will fall by the wayside in this group, with Paris Saint-Germain, Manchester United and RB Leipzig all battling it out for the two qualifying spots.

PSG, with the firepower of Kylian Mbappe and Neymar, will be the team to beat, but United's front three of Marcus Rashford, Anthony Martial and Mason Greenwood also have goals in them. Leipzig will miss Timo Werner's goals following his summer move to Chelsea, but they have held onto defensive rock Dayot Upamecano.

Istanbul Basaksehir, making their first appearance in the Champions League, will be an awkward opponent too and qualification could depend on which teams take most points from the Turkish champions.

TO ADVANCE: PSG, Man United

MUST-SEE MATCH: PSG and RB Leipzig is a reprise of the one-legged semifinal this summer, which the French side won with ease thanks to goals from Marquinhos, Angel Di Maria and Juan Bernat. Will it be any different this time around?