A half-time tweet from Lee Dixon, an Arsenal man who won league titles in three decades and knows what is required, cut to the chase. “This is it. The season right here right now. 45 minutes. Man up.” His sentiment was piercingly accurate, as Arsenal’s players had stumbled through the first half in a manner that mostly veered between inhibited and inept.

Could they get a grip? Could a group of experienced players, recoiling at the sight of an enthusiastic yet makeshift United as if the opposition were superhuman, pull themselves together?

The answer – an emphatic no – left them reeling. The top three going into the weekend all endured challenging games for one reason or another, but while Leicester City and Tottenham Hotspur summoned the resources to grasp three more precious points, Arsenal caved in.

It was not so much seize the day as seize up. Frozen by the stress of the situation? It appeared that way as a sense of dysfunction infected every department in their team.

Defensively, they had moments of chaotic panic, summed up by the sight of Laurent Koscielny and Gabriel Paulista looking aghast at one another after the teenager Marcus Rashford ghosted between them with fine instinctive movement to score his second goal in three thrilling minutes. Arsenal’s rearguard on the day was confused, edgy and painfully inefficient.

The midfield core, with Aaron Ramsey partnering Francis Coquelin, does not work smoothly and lacks the subtlety that has been sorely missed since Santi Cazorla got injured.

As for the attack, which has been a problem in recent weeks as the goals have dried up, Danny Welbeck scored and put in a decent shift but the other attackers are severely short of scoring form. Alexis Sánchez cannot find his rhythm, Theo Walcott’s lack of contribution and effort was embarrassingly shown up by the young English striker wreaking havoc for the other team, and the substitute Olivier Giroud is struggling for goals.

Arsenal looked anything but a team going for the title – unless that title is for the world choking championship. A tendency to bottle high-pressure opportunities is an accusation Arsène Wenger has faced more regularly than he would care to mention in the post-Invincibles years, and here it was once again, to his obvious frustration.

Despite the broadening gap at the top, with Arsenal now trailing Leicester by five points and Tottenham by three, and a significant goal difference disadvantage, Wenger refused to throw in any towels in terms of the Premier League. “We have not to feel sorry for ourselves,” he insisted, urging his team to bounce back on Wednesday when they host Swansea City. Following that, ominously, they travel to White Hart Lane.

Both those matches demand a huge amount of nerve, which is exactly what Arsenal currently lack. The margin for any more errors is wafer thin, which is a situation that tends to add to the tensions that gnaw at their confidence and pick at their composure.

The teamsheet at Old Trafford suggested a story of men against boys, an invitation to grasp the initiative. Yet the men wilted as the boys shone. What a moment for Louis van Gaal, on the back of an upbeat week of cup football, to see so much positive energy flowing in a challenge of this nature. The emergence of Rashford, debuts from the bench handed out to two more teenagers, and a sound display from the inexperienced Guillermo Varela ignited a fresh sense of hopefulness.

This season for United may not go down as one of the greats but it has had its unforgettable moments. Old Trafford has enjoyed – more than once now – the shock of the new as a youthful talent blooms in front of their eyes. On a September evening, on his Premier League debut, Anthony Martial inspired a massed gasp of amazement to score an electrically charged goal, against Liverpool, by way of introduction. That was emulated here by Rashford, the 18‑year‑old rookie who stunned Arsenal in his maiden Premier League appearance.

Every time he surged forwards, full of speed, bristling with enthusiasm and intent, a sense of panic coursed through the visiting defence. Rashford left them in a blur. Disorientated. And ultimately, badly defeated.

All that audacity, fearlessness and excitement contrasted with Arsenal’s angst and disappointment. When they needed to step up their limitations and foibles were exposed.

In answer to a question about Van Gaal’s amateur theatrics in the technical area, the Arsenal manager preferred not to comment. “He will say I am a bitter loser,” he said, before candidly adding, “I am, in fact.”

If there is any chance of salvaging their damaged title hopes, Arsenal’s players need to be the same. They need to feel defeat so bitterly it jolts them into manning up and finding some form and cohesion before it is too late. If it isn’t already.