William Saliba has played more than a century of matches in an Arsenal shirt and, in all of those hours of action against all of those different opponents, he has never looked as deeply uncomfortable as he did against Alexander Isak on Wednesday night.

Gabriel Magalhaes has been alongside Saliba for almost all of those matches and he, too, has rarely looked as flustered as he did in the first half of the League Cup semi-final second leg. Even Declan Rice, Arsenal’s midfield monster, found himself on the other side for once. This time, at the key point of the second half, it was Rice who was monstered.

Those three players, along with Thomas Partey and goalkeeper David Raya, constitute Arsenal’s defensive spine. For so much of the past few years, that spine has been powerful and strong. Against a rampant Newcastle, and one of Europe’s finest strikers, it cracked. Newcastle’s second goal of the night, their fourth of the tie, was the ultimate demonstration of how the home side were able to rock Mikel Arteta’s team.

Centre-back Fabian Schar had the courage to follow Rice all the way to the edge of Arsenal’s penalty area, and then the defensive quality to snatch the ball away, allowing Anthony Gordon to score. It was a remarkable incident from Arsenal’s perspective, primarily because it was so unusual. Raya, playing an erroneous pass so close to his own goal? And Rice, losing an important challenge in such a perilous position?

Such things almost never happen. Usually, as it was against Manchester City on Sunday, this is the kind of pain that Arsenal inflict upon their opponents. “It’s true that today we were a bit more unsettled,” said Arteta. “Especially when the game became more chaotic or when they were very direct with their play. Normally we are much more composed and today we lacked a bit of that, and the game went away from us.”

Just as eye-catching as Gordon’s goal was Isak’s first-half performance against the duo of Saliba and Gabriel, who have been probably the most consistent defensive pairing in the Premier League over the past 18 months. Isak stretched them, ran in behind, dropped into midfield and instilled a level of fear that must have felt entirely unfamiliar to the two defenders.

It is a measure of Isak’s brilliance that these same two defenders came up against Erling Haaland, widely regarded as Europe’s best centre-forward, on Sunday – and endured none of the same anguish. Haaland did score in the second half, yes, but in general play he was completely nullified. Isak, by contrast, was not under Arsenal’s control at any point. After the first leg, in which Isak scored one goal and created another, the main talking point among Arsenal supporters was that they would love to have him in their side.

This second leg would only have strengthened those feelings. Indeed, such is the individual quality of Isak (ably supported by the excellent Gordon) that Saliba and Gabriel might not actually take too many concerns out of this game and into the club’s Dubai training camp. Sometimes, you simply must accept that the opponent in front of you is operating on a higher plane of footballing existence.

Thankfully for Arsenal, they do not play against Isak at St James’ Park every week. If Arsenal can have their way in the summer transfer window, they might never play against Isak at St James’ Park ever again. On a wider scale, though, it will be a source of enormous concern to Arteta that his team were so unable to impose their style of play in this most ferocious of atmospheres.

If Arsenal are to go all the way in their pursuit of silverware, they need to not only survive such environments but thrive in them. They have shown in recent seasons they are capable of doing it, but they now need to be able to do so consistently, big game after big game. Their spine, so solid for so much of the past 18 months, needs to remain strong under the most intense pressure. On this occasion, it did not.