We look at the big issues ahead of Tuesday's matches

Show caption: Michael Carrick's Manchester United have gone nine games wit…

The Uefa Champions League is back in business this week as the group stage reaches Matchday 4, with nine fixtures taking place on both Tuesday and Wednesday.

Back-to-back champions and current table-toppers Paris Saint-Germain are among the clubs playing on Tuesday when they travel to Villarreal, looking to maintain their perfect start.

Barcelona, who lost 4-2 to PSG last time out, take on another side, Aston Villa, who have a 100 per cent record, at Camp Nou, while there are two other heavyweight clashes when Atletico Madrid play host to Bayern Munich. Manchester United face Roma at Old Trafford having not won in nine matches.

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Below, we pick out the main talking points ahead of Tuesday's fixtures.

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Barcelona banging in goals

Barcelona's flying start was brought to a grinding halt with the defeat to PSG followed by a 2-1 loss to Real Madrid in El Clasico.

But Hansi Flick's side have been prolific in front of goal this season. On Saturday, Barca smashed six past hapless Alaves, which saw Raphinha and Lamine Yamal both hit doubles, taking the Catalan club's La Liga tally to 46 goals in 11 games. Add three Champions League games into the mix and the total jumps to a breathtaking 57.

Brazilian forward Raphinha leads La Liga's scoring chart with 15 goals in 10 games – the most of any player in Europe's top five leagues – while Spanish winger Yamal has 10 goals to go with his top-flight-leading seven assists.

“Raphinha? I can’t be more happy,” said manager Flick on Monday. “He is delivering incredible performances. He's a very focused footballer, our captain, and he's showing his huge quality. That's very important for us”.

Emery rebuilds Villa

Next up for Barca will be English Premier League side Aston Villa, who will have a familiar face in the dugout at Camp Nou. Unai Emery won four Europa League titles while in charge of Sevilla and Villarreal, earning a fifth crown with Villa last season.

It has been a summer of rebuilding for the Spanish coach after seeing the spine ripped out of his successful team with Emi Martinez, Ezri Konsa, Youri Tielemans, Morgan Rodgers and Ollie Watkins all heading for the exit, bringing in more than £300 million in sales.

In the other direction, the likes of Zion Suzuki, Joao Gomes, Johan Manzambi, Ibrahim Mbaye and Nicolas Jackson all arrived in the Midlands - for a total of around £250m - but took time to settle as Villa took one point from their opening four Premier League games.

But recovery is well underway as Villa have now gone five games unbeaten in the league while winning three out of three in Europe. “Each competition is giving us a lot of moments to build the team. We are building and we are working to improve,” said Emery after Saturday's draw with Fulham.

Bayern navigate Kompany absence

Last season's semi-finalists Bayern Munich are looking for a return to winning ways when they face Atletico Madrid in the Spanish capital, after two consecutive draws in the Champions League.

It has been a tough few weeks since the German double winners drew 2-2 with Arsenal after manager Vincent Kompany collapsed at the club's training ground, which was diagnosed as meningeal irritation – an inflammation of the protective membrane surrounding the brain and spinal cord.

The Belgian has since been discharged from hospital, but will be absent from the dugout for “weeks, not days” according to Bayern's director of sport, Max Eberl. “Given the circumstances, he's doing well.”

In his absence, Bayern have won 3-1 away to Freiburg, hammered Magdeburg 6-1 in the German Cup, before drawing 2-2 with title rivals Borussia Dortmund on Saturday, with newly crowned Ballon d'Or winner Harry Kane missing a penalty in the latter.

Carrick's Man United long goodbye

Michael Carrick's seemingly inevitable Manchester United exit is turning into the most painful of long goodbyes. On Saturday, Carrick watched his side capitulate to a 2-0 defeat at Chelsea, taking their winless streak across all competitions to nine matches – their worst run since 1986.

Lisandro Martinez was sent off at Stamford Bridge after picking up two bookings in three minutes, the second of which also conceded a penalty – after inexplicably thrusting his arm into the face of Wesley Fofana – which Cole Palmer converted for the killer second goal. The defeat leaves United just outside the bottom three.

After their previous two Champions League defeats – at Atletico Madrid and then Como – both Martinez and captain Bruno Fernandes stuck their boot into the club bosses, piling the pressure on a beleaguered Carrick, who has cut an increasingly dejected figure in recent matches.

“The commitment of the players to the club, to me, to each other – do not question at all,” said Carrick ahead of the Roma game. “We need to play better, we can make better decisions, but the commitment is not a thing at all.”

Malen the man for Roma

Standing in the way of a desperately needed victory for Carrick are Roma, who go into the game on the back of a 5-2 Serie A demolition of Udinese on Saturday.

Gian Piero Gasperini's side are in fine form in front of goal, having scored 16 goals in their past four games across all competitions, including a 4-0 thrashing of Yaya Toure's Slovan Bratislava to earn their first Champions League win of the campaign.

Donyell Malen – whose loan move from Aston Villa was made permanent over the summer – continues to lead their goal charge with 11 in 12 matches this season, taking his Roma tally to 26 goals in 32 games.

“It is extraordinary, the statistics around Malen,” said Gasperini of his Dutch forward. “I knew that signing Malen would be an important step for us, but he's gone well beyond any expectations.”