We are midway through the Champions League group stages for the 2020-21 campaign and there are plenty of talking points as we enter into more midweek action.

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Premier League clubs Manchester United and Chelsea could take huge steps towards the knockout stages as Liverpool continue to deal with their injury crisis, whilst reigning champions Bayern Munich could re-write the history books and Barcelona will have a rather noticeable absentee.

Here, Sportsmail looks at 10 talking points ahead of this week’s Champions League action.

missing for Barcelona

Lionel Messi has been left out of the Barcelona squad to face Dynamo Kiev on Tuesday after one of his worst performances for the club in Saturday’s defeat by Atletico Madrid.

Whilst the omission of Messi could be a sign of further distress between him and the club, manager Ronald Koeman cited the reasoning for leaving him and midfielder Frenkie De Jong out is due to their comfortable position in the group.

Barcelona are top of Group G, seven points ahead of third-placed Kiev so allowing Messi to rest makes sense on the surface, but add more context and Barca fans could be getting a taste into the near future as Messi continues to be linked with a move away.

The Argentine has not looked himself this season. He lost the ball an astonishing 23 times in the 1-0 defeat at the Wanda Metropolitano. He also managed just 36 passes.

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Four-goal Haaland on fire

Borussia Dortmund frontman Erling Haaland continues to prove why he is one of the most sought-after strikers in the world following his Golden Boy award triumph, naming him as Europe’s best player under the age of 21.

The Norway international, 20, currently leads the Champions League golden boot race alongside Liverpool’s Diogo Jota, Juventus striker Alvaro Morata and Manchester United’s Marcus Rashford.

He will likely lead the line as Dortmund play host to Club Brugge after he scored an incredible four goals in 22 minutes against Hertha Berlin on the weekend.

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Manchester United’s revenge mission

’s most recent memory of the Champions League will be one of his worst since he took over as Manchester United manager.

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The Red Devils host Istanbul Basaksehir on Tuesday and revenge must be on the agenda. United suffered a shock 2-1 defeat by the Turkish side in the reverse fixture and Demba Ba’s opener highlighted their shambolic defensive organisation.

It’s still all to play for in Group H with United and RB Leipzig on six points but not far behind them are Basaksehir and last year’s finalists Paris Saint-Germain on three points.

Chelsea can secure progress

Frank Lampard can lead his side to the knockout stages of the Champions League should they beat Rennes and Sevilla win against Krasnodar in Group E’s other match-up.

The Blues are starting to find their feet under Lampard with all their new summer additions having had a little more time on the training pitch.

Entering the match unbeaten in nine matches across all competitions and with five straight victories, including four clean sheets, Chelsea will want to assure their place in the next round as it will allow them to rest key players for the final two fixtures in the group as part of this season’s hectic schedule.

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Who will play for injury-stricken Liverpool?

Jurgen Klopp finds himself at sixes and sevens trying to assemble a squad for the next few matches as Liverpool’s injury crisis continues.

The Reds are without Trent Alexander-Arnold, Joe Gomez, , Jordan Henderson, Naby Keita, Alex Oxlade-Chamberlain and Xherdan Shaqiri, with Thiago Alcantara and Rhys Williams still fighting for fitness ahead of Wednesday’s Champions League tie with Atalanta.

Fortunately for Klopp, he has a pretty full compliment in attack with Mohamed Salah returning after he recovered from coronavirus to join up with the in-form Diogo Jota, and Roberto Firmino.

The Reds will have to rely on young talents such as Curtis Jones, who has shown plenty of promise under Klopp, as they navigate their way through the winter’s busy schedule without so many of their star players.

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Real Madrid and ’s must-win match-up

Group B would look more realistic if it was turned upside down, but the Champions League loves to throw up surprises.

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Real Madrid and Inter find themselves sitting third and fourth respectively in the group and now face each other in a must-win match on Wednesday.

The Italians are yet to win a match this campaign, with Antonio Conte’s side managing just two draws and one defeat. Meanwhile, Madrid are making hard work of their group, also consisting of Borussia Monchengladbach and Shakhtar Donetsk, with a win, draw and loss.

Due to their position, both teams will be desperately fighting for the win as a draw would help neither. But a defeat would leave them with an almighty uphill battle against an embarrassing expulsion.

Man City must get clinical

Pep Guardiola will still be scratching his head over City’s barren trip to the Tottenham Hotspur Stadium on the weekend.

Despite having 22 shots against Spurs, they failed to score and lost 2-0 to the current Premier League leaders.

Summer signing Ferran Torres could be crucial to City’s goal hopes as he netted in their reverse fixture against Olympiacos, their opponents on Wednesday, and scored a hat-trick during the international break for Spain in their emphatic win over Germany.

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Bayern can continue to re-write the history books

The reigning champions have continued to assert their European dominance this season as they lead the way in Group A with three victories from three matches.

On Tuesday they face strugglers Red Bull Salzburg, who they beat 6-2 in the reverse fixture, and can also extend their run of consecutive Champions League wins to 15.

Bayern broke the record for most consecutive wins in the Champions League last season, surpassing Real Madrid’s 10 straight wins in the 2014-15 campaign, and with every win since have re-written the history books.

Whilst they are missing key players and Joshua Kimmich through injury, the Bundesliga champions should still cruise to another win and secure qualification to the next stage of the competition.

Even if Salzburg hold out strong, Bayern have proven they can break teams down eventually as striker Robert Lewandowski has scored in the final 15 minutes in each of his last five goalscoring European appearances.

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PSG face embarrassing exit

After coming so close to winning the Champions League last season, PSG find themselves in danger of dropping out of the competition altogether in the group stage.

In their way this week is RB Leipzig, who beat them in the reverse fixture and left them with just one win from three matches in Group H.

PSG’s heavy funding has always been with a view to becoming European champions given their resources were already strong enough to establish themselves as the best club in France.

With Tuesday’s result having so much riding on their group standing and a potentially embarrassing exit if it goes wrong, you have to feel that the match could be defining for manager Thomas Tuchel and talisman Kylian Mbappe regarding their future in Paris.

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Monchengladbach and Shakhtar’s unlikely supremacy

As mentioned earlier, Group B has brought the most surprises as Borussia Monchengladbach lead the way just ahead of Shakhtar Donetsk.

Behind them, Real Madrid and Inter Milan will fight for a huge three points as whoever wins in this match-up could deal a crushing blow to the loser of the other match and potentially demote them to the Europa League.

The Germans have been on form in this competition with 10 goals from their opening three matches, with striker Alassane Plea sitting on three goals and three assists.