As a boy Joe Ironside relished watching his idol, Alan Shearer, play centre-forward for Newcastle United.

Ironside was back at St James’ Park wearing Cambridge United’s No 9 shirt and he suggested he had learned a thing or two from all those afternoons spent studying Shearer’s movement.

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In the 56th minute the Middlesbrough-born striker scored the sort of goal his role model would have treasured to not merely book Mark Bonner’s outstanding League One side a fourth-round place but subject Newcastle to the sort of giantkilling that makes the FA Cup so special.

Cambridge are 41 places beneath Eddie Howe’s team on the league ladder but you would not have known it for much of a tie that ended with Newcastle still having won only one game all season and Bonner’s players cavorting across the pitch, hugging everyone in sight.

Earlier it had seemed fate might be being tempted when, around the 10-minute mark, the first choruses of “Premier League, you’re having a laugh” drifted down from the vertiginous top tier of the Leazes End that housed 5,000 travelling Cambridge fans.

Their team had started well, defending with industry, intelligence and compactness as they persistently second-guessed the intentions of the theoretically strong Newcastle starting XI. Although Kieran Trippier raised the tone courtesy of some high-calibre touches on his debut, Howe’s side played with a distinct, and disturbing, uncertainty.

After registering only one victory since August, Newcastle frequently did not seem to trust their ability and instincts, often taking a touch too many or delaying a pass a couple of seconds too long. One of the few players who seems immune to such introspection and knows better than to overthink things is Allan Saint-Maximin but even Howe’s great improviser frequently found himself cleverly shadowed by his League One minders.

Moreover Saint-Maximin suffered the indignity of, on more than one occasion, being all too easily brushed aside by Bonner’s captain, Paul Digby. Deployed in a defensive midfield role, Digby served as a quasi sweeper, screening the visiting backline superbly.

With the home passing radar awry and pressing game malfunctioning, Cambridge sensed opportunity and Ironside troubled Howe’s back four with a mix of crafty movement and edgy presence.

Even at this relatively early stage Howe cut an uneasy figure in the technical area. If Trippier’s tone-raising cameos probably afforded him a degree of reassurance, it became increasingly clear that, good as the England right-back undeniably is, this was most definitely not the sort of debut he dreamed of when he completed his £15m move from Atlético Madrid on Friday.

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Trippier said he likes “a challenge” and in signing up to help fix the appreciable disconnect between Newcastle’s current travails and the Saudi Arabian-owned club’s new found status as theoretically the world’s wealthiest, Howe’s new full-back has certainly embraced one.

Normally, Trippier’s right-wing deliveries are stellar but here they seemed to persistently hit a wall in the formidable shape of Bonner’s commanding centre-back Jubril Okedina and his fellow defenders.

On those inevitable occasions when Okedina and co were breached, the excellent Dimitar Mitov made some important saves, most notably as, late in the first half, Newcastle enjoyed their best period of the tie.

Indeed Cambridge’s goalkeeper performed acrobatic wonders to twice deny Jacob Murphy and then Saint-Maximin after the latter, albeit temporarily, began confounding his markers. The moment Mitov somehow tipped Murphy’s volley on to the bar will linger in the memory.

At the other end though, Ironside retained a capacity to unnerve Emil Krafth and Fabian Schär. Bonner appealed for a legitimate looking penalty when Harvey Knibbs collapsed in the area after appearing to be tugged back by Matt Ritchie but, contentiously, a VAR review revealed no foul.

Schär is often much better at creating than deconstructing and the Switzerland defender momentarily thought he had scored after tapping in the fallout from a Ritchie corner but was well offside.

Murphy, too, had an effort disallowed for offside early in the second period but it was not long before Ironside showed the stunned home fans precisely how it should be done.

When Newcastle failed to clear a loose ball, in the wake of Schär heading Adam May’s shot off the line, Bonner’s key striker pounced, swivelling sharply before lashing the ball beyond the helpless Martin Dubravka. Newcastle’s goalkeeper injured himself in the process, playing on in apparent pain.

After an inordinately long VAR check for a possible offside, the goal stood and from then on Cambridge shone, with Bonner’s narrow configuration resembling the sort of insurmountable yellow barrier certain to stalk Howe’s nightmares.

Newcastle’s manager looked haunted when Jack Lankester shot beneath Dubravka but he had strayed fractionally offside and it was disallowed. No matter; Ironside had already done enough.