Matchday 4 of the Champions League group stage saw round-of-16 places clinched by Liverpool, Bayern Munich, Juventus and Ajax, while Manchester United conjured another late show and there were narrow Barcelona and Real Madrid wins, but Paris Saint-Germain slipped up and Atletico Madrid and AC Milan continued to struggle.

We asked James Olley, Tom Hamilton and Andrew Richardson to answer some big questions.

What caught your attention on Matchday 4?

Olley: The curious relationship Liverpool fans have with Luis Suarez. His 59th-minute substitution during Wednesday's 2-0 win over Atletico Madrid was belatedly greeted with muted applause but the home crowd had long had their fun as booing was heard. Suarez scored 82 goals in 133 appearances during three seasons with the club to 2014 but was dogged by controversy, twice being banned for biting an opponent (once on international duty with Uruguay) and serving a further suspension for racially abusing Patrice Evra during a match against Manchester United in 2011.

In fairness, Suarez emphatically celebrated a goal for Barcelona against Liverpool during their Champions League semifinal first leg in May 2019 and he spent a fair portion of the reverse fixture against Atletico kicking lumps out of Virgil van Dijk. But it was almost as if Reds fans enjoyed an opportunity to cast him as a pantomime villain. Liverpool's players infamously wore t-shirts in support of Suarez as an investigation into the Evra incident took place. On Wednesday night, one fan behind the press box could be heard calling Suarez a "racist." How times change.

Hamilton: Ajax beat Borussia Dortmund 3-1 to become the first Dutch side to win their opening four group games and are playing with the same confidence that took them to the semifinals in 2018-19. Moreover, the club's brilliant recruitment was there in red and white in the Westfalenstadion, with Sebastien Haller and Dusan Tadic grabbing a goal each.

Haller was unfancied at West Ham, but has been a revelation with seven goals in this season's group stage and Tadic, picked up for £12 million from Southampton in 2018, could lay claim to being Ajax's best player of the 21st century. Meanwhile, Erik ten Hag's stock continues to rise as one of Europe's best coaches, but this win was more proof of what can be achieved when the whole machine works in harmony.

Elsewhere, Sporting Lisbon were brilliant to watch with their all-out attack approach in the 4-0 victory against Besiktas, while Trent Alexander-Arnold was magnificent for Liverpool in the 2-0 win over Atletico Madrid. And if you want a performance of the round, look no further than Paulo Dybala's showing for Juventus in the 4-1 win against Zenit.

Richardson: Atalanta centre-forward Duvan Zapata showed he is comfortable playing at Champions League level against a Manchester United backline containing one of England's best defenders (Harry Maguire) and a four-time winner of the competition (Raphael Varane).

Zapata has had moments in Serie A, but to star on the biggest stage against one of the biggest clubs in the game is special. He may not score tons of goals and is already 30, but the Colombia international keeps defenders busy and creates space for others. Atalanta may struggle to keep hold of him.

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Another Man United late show, but can Solskjaer turn them around?

Richardson: No chance. Ole Gunnar Solskjaer is completely out of his depth and, while Cristiano Ronaldo has saved the manager's job (as well as vital points) for three successive Champions League matches, it is completely unsustainable.

United's squad has some real talent -- a mixture of experienced superstars, exciting young players and others in their prime -- yet that talent is being wasted by a manager who really has no idea at this level. Total dysfunction in the hierarchy above him is surely the only reason Solskjaer is still manager.

Olley: United will find a way to qualify for the knockout stages but they were hugely fortunate to take four points from the two games against Atalanta. And so the Solksjaer cycle continues: one game away from disaster, pulled back from the brink by individual quality yet lacking any signs of working towards a collective gameplan that can make them more impervious to fluctuations in form. When the lack of an obvious successor is arguably the main thing keeping you in a job, the writing really is on the wall.

Hamilton: United looked like an abstract group of world-class players against Atalanta, rather than a team, but that is symptomatic of the club's malaise and confusion. They have the players -- bar a ball-winning midfielder -- but neither the gameplan nor structure.

They want to play attacking football at Old Trafford, built on the foundations of a superb academy, so if the owners have any title-challenging ambition then they need to learn from what Ajax have mastered over so many years.

With two games to go, a big club that will not make the round of 16 is...

Hamilton: Barcelona. Even though they won at Dynamo Kiev -- thanks to the genius of Ansu Fati -- they will struggle to get through Group E. Given they have a trip to Bayern Munich on Matchday 6, Barca simply have to get three points against Benfica next time out, or the advantage will be with Jorge Jesus' team.

Richardson: AC Milan. No matter how unfancied they may be coming into a particular campaign, the Rossoneri are a footballing institution and always expected to deliver, such is their relationship with the competition they have won seven times.

Facing a group with Liverpool, Atletico Madrid and Porto was daunting, true, but while they were not a favourite to win the Champions League, Milan's most recent triumphs have come at unexpected times. Moreover, they are joint-top of Serie A, so to go out at the group stage is a surprise and a disappointment.

Olley: Andrew's right about AC Milan but Atletico Madrid might be in a little trouble too. The problem they both have is that Liverpool have already secured top spot in Group B and Jurgen Klopp is therefore highly likely to rotate his line-up for the next game at home to Porto, which will not quite be the same daunting task that Porto would have originally anticipated.

Milan host Liverpool in the final round of matches, too, so Atletico will be disadvantaged should Klopp rest his stars once again. Atletico will also have to contend with suspensions for Antoine Griezmann and Felipe.

Which young players, from unfancied teams, can play at the top level?

Olley: Not sure Ajax count as an unfancied team given their pedigree but there is a reason why Erik ten Hag keeps resisting a move to the Premier League and it could well be the quality of the next generation coming through. Antony, a highly promising winger, could yet beat him to England given reported interest from Manchester City and Liverpool. The 21-year-old Brazilian is lightning quick and plays with a swagger that suggests a career at the highest level awaits.

Hamilton: Karim Adeyemi is the next star tipped for a move from FC Salzburg to a European giant. He does not turn 20 until January, but a series of superb performances in the Champions League have led to reported interest from PSG, Bayern Munich and Borussia Dortmund, as well as international recognition with Germany.

But the Austrian club, where Erling Haaland and Takumi Minamino burst onto the big stage, has more talent in its midst, with 21-year-old midfielders Nicolas Seiwald and Mohamed Camara worth watching. Seiwald ranks second for most presses in the group stage with 117, while Camara is a fine tackler and is top 10 for ball recoveries (57).

Richardson: Brahim Diaz won't be in the Champions League knockout rounds with AC Milan this year but is likely to be a regular in seasons to come. On loan from Real Madrid, he could become a mainstay in the Los Blancos lineup if he continues to play to this level. Has been immense this season and is a worthy wearer of the No. 10 shirt.