Only one team in Champions League history has previously won back-to-back titles.

Real Madrid's all-conquering team, led by Zinedine Zidane, won club football's biggest prize for an unprecedented three consecutive years between 2016 and 2018 -- now Bayern Munich has a realistic chance of doing the same.

Few could argue that the perennial German champion was not the best team in Europe last season and a deserving Champions League winner. Hansi Flick's side returned from the coronavirus-enforced break in unstoppable form, winning every remaining game of the season.

The most eye-catching victory in the record-breaking 23-game winning streak -- which began against Cologne back in February and ended with defeat to Hoffenheim at the end of September -- was the 8-2 humiliation of Barcelona in the Champions League quarterfinals.

It's also been estimated that Bayern could earn over $150 million for winning last season's tournament.

Perhaps most concerningly for Bayern's European rivals is that the Bavarian club has arguably only gotten stronger in the off-season. Despite losing midfield maestro Thiago to Liverpool and Philippe Coutinho returning to Barcelona, Bayern has bolstered its ranks with the additions of Leroy Sane from Manchester City and highly-rated Spanish youngster Marc Roca, as well as the return of winger Douglas Costa.

Bayern has a tricky but manageable group to kick off the defense of its title. Diego Simeone's Atletico Madrid will likely be a contender for top spot, followed by exciting Austrian side RB Salzburg, which put in some thrilling performances in the group stages against Liverpool and Napoli last season.

Russian side Lokomotiv Moscow -- appearing in its first Champions League since 2003-04 -- completes the group.

Messi vs. Ronaldo

A lot has been made of Lionel Messi's motivation going into this season ... or lack thereof, more specifically.

The Argentine made public his desire to leave Barcelona at the end of last season and believed a clause in his contract allowed him to do so for free.

However, Barcelona refused to budge on the Argentine's $897 million release clause and, not wanting to drag the club he says he loves into an unsavory legal dispute, Messi opted to stay.

Despite the off-field distractions -- of which there are plenty at the Catalan club -- Barcelona started the La Liga campaign in good form with Ronald Koeman as its new head coach, winning its opening two games before drawing against Sevilla.

However, several of the team's flaws reemerged over the weekend as an insipid performance saw Barcelona suffer a surprise 1-0 defeat to Getafe.

But Ryan Babel, who played under Koeman during his two-and-a-half years in charge of the Netherlands, believes the 57-year-old has the ability to turn the club's fortunes around.

"He's a very good coach," he told CNN. "However, sometimes in football timing is very important. Right now, there's a lot going on in Barcelona. I just hope that it doesn't affect the process of him focusing on shaping his squad.

"He's a very good coach and able to bring success back to Barcelona. For me, I think one of his strengths is giving players responsibility and creating a group process ... a certain atmosphere that he's really building a team, even here. For us, Koeman created an atmosphere that everyone wanted to be a part of."

Messi has only managed one goal so far, a penalty on the opening day against Villarreal, but Barca has been buoyed by the performances of exciting youngster Ansu Fati, who has bagged three goals in the opening three games and also made history as Spain's youngest ever goalscorer during the international break.

To make matters even more intriguing will be the presence of Juventus in the same group, with Messi and Cristiano Ronaldo going head-to-head in the Champions League group stages for the first time.

Juve, too, has its fair share of concerns heading into this year's campaign, with talisman Ronaldo set to miss out against Dynamo Kyiv on Tuesday and a doubt for the highly-anticipated clash against Barcelona on October 28 due to a positive coronavirus test.

With Juventus and Barcelona vying for top spot, spare a thought for underdog rivals Dynamo Kiev and Ferencvaros, who will no doubt both be hoping for that third Europa League spot.

Best of the rest

Paris Saint-Germain, last season's beaten finalist, has been drawn in a tricky group alongside Manchester United, RB Leipzig, and Turkish champion Istanbul Başakşehir.

The French champion has endured a difficult start to the domestic campaign, as a host of coronavirus cases and a rushed start to the Ligue 1 season saw it lose the opening two matches, before bouncing back to go on a five-game winning streak.

Despite bolstering the squad with the likes of Danilo Pereira, Rafinha and Alessandro Florenzi, PSG is yet to sign a replacement for captain and vastly experienced center back Thiago Silva, who left to join Chelsea in the transfer window.

The club came so close to clinching its first European Cup, the trophy its Qatari owners crave the most, and it remains to be seen whether last season's final defeat will deflate the squad or become an added motivation.

Meanwhile, despite having never won the title, Manchester City once again enter into this season's competition as the bookmakers' joint favorite.

Pep Guardiola's side was stunned by Lyon in last season's quarterfinal and the Spanish manager is still looking for his first Champions League triumph since leaving Barcelona in 2012.

Liverpool, champion in 2019, has endured a turbulent start to the Premier League season. After winning its opening three games, two of those in impressive fashion, Jurgen Klopp's side was humiliated 7-2 by Aston Villa, a side which escaped relegation last season by one point.

Liverpool now has the joint worst defensive record in the league -- for goals conceded -- and cracks that had started to appear last season offer opposing teams plenty of opportunities to exploit, notably in the way Villa preyed on the Reds' high defensive back line.

Key defender Virgil van Dijk suffered a knee ligament in the 2-2 draw against Everton on Saturday and will be out for lengthy spell, further adding to Liverpool's defensive worries.

There will be no respite in the Champions League, either, with a tricky draw against two brilliant attacking teams in Atalanta and Ajax that could expose Liverpool's creaky defense. Time will tell whether this is just a short-term blip or a more chronic problem.