A look at what's happening around the Champions League this week:
Manchester City vs Napoli
Something has to give when the formidable attacks of Manchester City and Napoli meet in Group F on Tuesday.
The Premier League and Serie A leaders have been dominant in domestic football, scoring freely. The only time either team has failed to win was when City drew with Everton back in August.
In Europe, however, it is City which possesses the perfect record, having defeated Feyernood and Shakhtar Donetsk, while Napoli lost 2-1 when they travelled to Ukraine.
And so the Italian club is under pressure to get a result when it travels on Manchester to take on City at the Etihad Stadium.
City thrashed Stoke 7-2 in the Premier League on the weekend and that was without striker Sergio Aguero, who is likely to make his return from the rib injury he sustained in a car crash in Amsterdam last month.
City's sometimes-shaky defense will be tested though, with Napoli scoring at least three goals in all seven of its Serie A games before a hard-fought 1-0 victory away to rival Roma on Saturday. Napoli has won 11 of 12 matches overall this season.
Juventus vs Sporting Lisbon, FC Barcelona vs Olympiakos
Having reached the final twice in the last three seasons, Juventus appears to have taken a dip in form this year.
The Bianconeri have dropped points in two straight Serie A matches and made a series of errors in a 2-1 home loss to Lazio on Saturday.
The unexpected transfer of center back Leonardo Bonucci to AC Milan in July has highlighted the diminished impact of aging defenders Andrea Barzagli and Giorgio Chiellini.
Barzagli and Chiellini were at fault for both of Ciro Immobile's goals for Lazio.
Juventus also badly misses the influence of fullback Dani Alves, who was a key player last season and very popular among teammates. He has joined Paris Saint-Germain, where his astute passing and rampaging runs down the right flank are already proving a hit.
Mattia De Sciglio, signed from AC Milan, has struggled to fill the void left by Alves. Still, Juventus will be favored to win against Sporting Lisbon in Group D on Wednesday. Barcelona, which hosts Greek club Olympiakos, leads the group with six points, Juventus and Sporting have three each and Olympiakos has none.
Atletico Madrid vs Qarabag, Chelsea vs AS Roma
Atletico Madrid has thrived in the Champions League in recent years, but may not get past the group stage if it fails to win at Qarabag in Group C on Wednesday. Anything but a victory in Baku will leave Diego Simeone's team close to elimination after four straight seasons reaching at least the quarterfinals. It lost in the final to cross-city rival Real Madrid in 2014 and 2016. Atletico has only one point, trailing AS Roma by three and Chelsea by five. Qarabag should offer a limited threat, however, having lost both of its games so far.
Real Madrid vs Tottenham Hotspurs
Two-time defending champion Real Madrid can take the lead in Group H when it hosts Premier League Tottenham on Tuesday. Madrid and Tottenham have two wins each. Sevilla can keep its Group E lead when it visits Spartak Moscow on Tuesday. Sevilla has four points, two more than Spartak and Liverpool.
Bayern Munich vs Celtic, PSG vs Anderlecht; Borussia Dortmund in action
When Jupp Heynckes was last in charge of Bayern Munich, he led the club to a Champions League, Bundesliga and Cup treble in 2013.
Now the 72-year-old German is back after being coaxed out of retirement to replace Carlo Ancelotti, who was fired following Bayern's 3-0 loss at Paris Saint-Germain. Heynckes, who oversaw a 5-0 rout of Freiburg in his first game back on Saturday, will hope for another convincing performance against visiting Celtic in Group B on Wednesday to shake off any lingering uncertainty after the team's unimpressive start to the season.
"We still have a lot of work ahead of us," he said. After starting with losses to Tottenham and Real Madrid, Dortmund needs a win at APOEL Nicosia on Tuesday to kick start its campaign in Group H.
Peter Bosz' lineup lost its 41-game unbeaten record at home in the Bundesliga on Saturday, when Leipzig inflicted a 3-2 defeat.
Leipzig is now aiming for its first win in European competition when it hosts Porto on Tuesday in Group G.
Containing Paris Saint-Germain's strike force of Neymar, Kylian Mbappe and Edinson Cavani is already a big enough test for the Anderlecht defense on Wednesday.
But with PSG favored to record a third straight Group B win, Anderlecht has another threat to worry about: defender Thomas Meunier.
The Belgium right back has seven goals and seven assists in his last 10 games for club and country; including both goals in Saturday's 2-1 win at Dijon.
The attack-minded Meunier is becoming a vital part of the PSG lineup with his quick feet, accurate passing and eye for goal.
"I used to be a striker when I was younger," said Meunier, who scored three goals and set up four in a 9-0 win against Gibraltar in a World Cup qualifier on Aug. 31. "There's no better feeling than when you score."
He will not have fond memories, though, of playing against Anderlecht with his former team Club Brugge.
It was his late own goal that gave Anderlecht a 1-0 win against Club Brugge late in the 2013-14 campaign. Anderlecht went onto to win the league title that season.
Monaco welcomes back top scorer Radamel Falcao and key midfielder Fabinho for Tuesday's must-win home match against Turkish team Besiktas in Group G.
Falcao was rested for Friday's match against Lyon while Fabinho was suspended. Monaco lost 3-2.
Monaco reached the Champions League semifinals last season but has one point so far, while Besiktas leads with six and Porto has three.
