Fermin is the first Spanish player to score a hat-trick for Barca in the Champions League (David Ramirez/Soccrates/Getty Images)

Barcelona cruised to a 6-1 Champions League home win over Olympiacos -- but they definitely saw a few decisions fall their way.

With El Clasico against Real Madrid to come for Barca on Sunday, Fermin Lopez scored a hat-trick, Lamine Yamal converted a penalty and Marcus Rashford got a brace to make it four goals from three games in Europe's elite club competition so far this term for him.

With the score still at 2-0, Olympiacos did actually manage to get themselves back in the game (albeit briefly) thanks to a spot-kick of their own. Ayoub El Kaabi put that in after a VAR intervention that chalked off a header from the Moroccan striker but awarded the Greek side a penalty in the same review.

Shortly afterwards, they were reduced to 10 players with Hezze dismissed for a second yellow card in what seemed a soft decision -- but one that VAR could not review.

That saw Barca kick up a gear, and in the end it was a very comfortable night for Hansi Flick's side, who had seven graduates from their La Masia youth academy in the starting line-up.

The Athletic's Dermot Corrigan runs through the big talking points.

Despite the overwhelming scoreline, Barca do have defensive issues right now and they were on show again in the way Olmpiacos worked a way back into the match.

Centre-back Eric Garcia was caught out under a cross and used his arm to stop El Kaabi from having a free header six yards out. When the Moroccan striker claimed a penalty, the Barca defender pleaded his case to the referee, instead of following the play.

So El Kaabi was in even more space when ex-Wolves winger Daniel Podence recovered the ball and sent over another cross, which the centre-forward headed powerfully to the net, with Garcia's partner Pau Cubarsi also not close enough to do anything.

Then, there was an unexpected twist.

Olympiacos' celebrations were stopped by a VAR check which judged that Podence was offside before he went to retrieve the ball. But any relief for Barca was short-lived as Garcia's handball was also spotted and meant a penalty was awarded to the visitors, which El Kaabi coolly converted.

But just four minutes later, Olympiacos were really unhappy with the officials. Referee Urs Schnyder showed a yellow card to visiting midfielder Santiago Hezze, when Marc Casado fell to the floor clutching his face, after the two tangled.

That meant a red card as Hezze had been booked in the first half for a foul on Pedri deep in the Barca half. Replays suggested that Casado had exaggerated what looked like very minimal contact from Hezze's arm, but in this case the rules meant that VAR could not intervene.

Yet another refeering talking point came soon after Olympiakos went down to 10 men, when Rashford claimed a penalty.

The England international played a one-two with Fermin on the edge of the visitors area, and raced to get the return, touching the ball just as goalkeeper Konstantinos Tzolakis dived at his feet.

The Swiss referee immediately pointed for a goal kick, rather than a penalty. He was then surrounded by Barca players claiming a penalty, while Tzolakis and his colleagues were also protesting that no foul had taken place.

His VAR colleagues sent him to review what happened on the pitchside monitor and it was determined that it was a penalty. Yamal took the ball and calmly rolled the ball to the net to make it 3-1 to Barca.

That was Rashford's best moment of a game in which he was mostly playing as the team's centre-forward, not on the left side of the attack, where he has spent most of his Barca career to date.

But it wasn't his best moment.

He did not look so comfortable as the focal point of the attack, struggling to hold possession when the ball was played up to him with his back to the opposition goal.

Yet when he was switched to the left, play opened up for him more and he seemed more comfortable, breaking in behind the defence, and cutting inside to shoot on his favoured right foot for his goal. Poor Tzolakis was left floundering as the ball hit the net.

Flick named Barca's third youngest XI in a Champions League match, with his options limited by a busy match calendar and so many senior players absent through injury, including Robert Lewandowski, Raphinha and Dani Olmo.

Youngest of all was 17 year-old-Dro Fernandez, making his second ever Barca start -- and one of seven La Liga Masia graduates in the line-up.

Dro impressed Flick during pre-season last summer, scoring a fine strike in a 3-1 friendly win over Vissel Kobe in July.

But this was very quick progress, given that, last season, the Galician was not a regular starter for Barca's under-19s in the UEFA Youth League, although he did score twice in the 107 minutes he played across seven games in that competition.

Promoted to the Champions League stage on Tuesday at Montjuic, Dro did not look out of place at all on the left side of the forward line, linking well with his team-mates, knowing which positions to take up in attack and also showing the workrate off the ball which Flick demands.

His best moment was when playing a key role in his team's second goal. When Pedri sharply won the ball back near half way, it broke to Dro. He quickly turned, drove at the Olympiacos defence, and cleverly timed his pass to Fermin, who calmly cut inside and found the net for the second goal in his hat-trick.

It was no surprise that Dro was the first Barca player replaced, just before the hour mark, but that was more due to his age and inexperience rather than any problems with his display. He got warm applause from the Montjuic crowd on his way off, and looks set to feature plenty more in the future.

Fermin is more established at Barca, but still just 22, and he was one of three La Masia graduates involved in the opener. Alejandro Balde, also 22, carried the ball deep into the opposition half. Lamine Yamal, 18, dribbled into the Olympiakos penalty area but was denied by visiting goalkeeper Tzolakis. The ball fell back to Fermin, who squeezed a shot through the scrambling defenders trying to cover on the line.

For this hat-trick -- his first as a senior pro and the first hat-trick scored for Barca by a Spanish player in the Champions League -- Roony Bardaji showed super skill to break down the right side of the area, the cross was deflected into Fermin's path, and he hammered an unstoppable left-foot shot to the net.

It was excellent timing, given Barca's injury issues ahead of Sunday's Clasico. Dani Olmo and Robert Lewandowski are definitely out, and Raphinha and Ferran Torres are racing to be fit on time.

It means that Fermin looks almost certain now to start at the Bernabeu, and Madrid will have to be wary of his goalscoring threat from attacking midfield.

Barcelona 6 Olympiacos 1 - Fermin's hat-trick, VAR controversy and a great night for La Masia