It was a complex offside situation. Sadio Mane had converted the rebound, seemingly giving Liverpool a 1-0 lead at the Emirates, but a linesman's flag had thwarted him. Mane had been offside when the initial pass was played, but wasn't interfering with play. When he was interfering with play, in the second phase, he'd been level with Roberto Firmino as the Brazilian volleyed towards goal. Mane was onside, the goal should have stood.
From the freeze-frames on the replay, we were looking at the position of Mane, the position of Firmino, and the position of the Arsenal defence. But not, crucially, at the nature of the pass.
It came from right-back Trent Alexander-Arnold. This wasn't a swinging cross around the defence and in behind the opposition, or a hopeful lofted ball into the channel. No, instead Alexander-Arnold had checked inside, dribbled towards the centre of the pitch, and then dinked a perfect ball over the top, with his weaker left foot, to put two Liverpool attackers running through on goal. It's the type of wrong-footed pass that even the Premier League's most gifted playmakers might not have attempted, played to absolute perfection, by a 20-year-old right-back.
The last time Liverpool's academy produced a player who enjoyed a spell in the first team at right-back, it was Jon Flanagan, who featured prominently during Liverpool's near-miss of 2013-14, attracting attention with his storming tackle. He was, somewhat ironically, nicknamed the "Scouse Cafu".
If Alexander-Arnold is going to be compared to a legendary right-back, though, then it's not Cafu but instead Philipp Lahm. The German World Cup-winning captain was renowned as a right-back, but he was equally comfortable playing on the opposite flank, and was later converted into a central midfielder by Pep Guardiola, who believed the optimum approach was to put the most intelligent, gifted players in the middle. Lahm, as it happens, had played central midfield in Bayern's youth sides, and only shifted right because there were doubts about his level of physicality.
Alexander-Arnold might embark upon a similar experience. He was also a midfielder when rising through the youth ranks -- in truth, almost every top-class full-back was converted from a more attacking position at some stage -- and became a right-back only a couple of years ago, essentially, because he realised that's where opportunities would present themselves in the first team. The more he plays right-back, though, the more it appears he belongs in a different position.
Which isn't to say that Alexander-Arnold has been exposed in defence. Only the trip to Naples this season stands out as a difficult game, and there have been many more success stories. In the 3-0 Champions League quarterfinal victory over Manchester City last season, Guardiola clearly tried to target Alexander-Arnold, suspecting he was Liverpool's defensive weakness. Guardiola used a midfield diamond, one striker, and one winger -- Leroy Sane, playing high up against Alexander-Arnold.
Almost all City's attacking play went through Sane, and every time Alexander-Arnold stood firm. In fact, not only that, but when Manchester City worked their most presentable opportunity of the game, teeing up David Silva for what looked like a tap-in from a right-wing pull-back, Alexander-Arnold darted inside, leaving his direct opponent Sane, to provide a crucial intervention in the middle. It showed a great level of footballing intelligence to cover for his teammates.
Alexander-Arnold later suggested Sane is the toughest opponent he's faced, but you wouldn't have known it. Since then he's largely nullified Cristiano Ronaldo in a Champions League final and outplayed Neymar at Anfield, not merely by getting his foot in, but also by bursting past on the overlap and exposing the Brazilian's lack of work rate.
Plenty of talented young right-backs can tackle and sprint, but Alexander-Arnold stands out for his technical quality. There was that left-footed chip against Arsenal, there was a lovely outside-of-the-boot ball into Sadio Mane against West Ham earlier this season, and a well-weighted curling ball down the line for Mohamed Salah's opener against Fulham this month.
He also regularly attempts driven crossfield balls to the opposite flank, and has scored two goals in his last two games -- a driven, powerful side-footed effort into the far corner in England's victory over the United States, and then a dipping free-kick which curled into the net away at Watford. Alexander-Arnold boasts more technical quality than we've come to expect from a right-back, traditionally the less flashy, more solid cousin of the more exotic left-back.
Playmaking skills have become more important from full-backs over the last couple of years. Pundits still talk about the nature of "modern full-backs", expected to overlap and provide crossing qualities, but realistically that concept has become long-established.
What's becoming increasingly important is full-backs being able to play incisive passes from deep positions, against teams who form compact blocks in the centre of the pitch, denying central defenders and central midfielders time on the ball. The full-backs have a little more space, and usually a full view of the pitch.
That recent trip to Arsenal was an interesting game in this respect, with both sides' full-back pairings attempting ambitious passes into the channels on a regular basis. Alexander-Arnold is fully capable of bending clever balls around defenders into onrushing forwards, and in that respect shares a similarity with Cesar Azpilicueta, or perhaps even Kevin De Bruyne, who are entirely different players but have recently shown a fondness for curling balls from deep, centre-right positions.
Alexander-Arnold has also made headlines recently for daring to take on world chess champion Magnus Carlsen -- lasting 17 moves before the inevitable defeat. Along those lines, it's becoming clear that Alexander-Arnold isn't merely a rook, starting wide and waiting for opportunities to burst up the touchline, but that he is also capable of showcasing the neat trickery in tight positions of a knight, or the central playmaking potential of a queen. He's already a top-class right-back, but if he keeps playing like he has he might not stay there for long.