After mid-morning wind and rain that might have caused Noah a problem, the skies cleared and Bournemouth took apart a Manchester United team as amateurish as any of the iterations of the past decade or so.

Ruben Amorim’s men could not defend, retain the ball or finish. So in these three fundamentals they failed, to leave their head coach appearing hapless and isolated. It is a fate familiar to all who have occupied the Old Trafford hotseat during this post-Sir Alex Ferguson era.

There was a high comedic element too. Before kick-off United were drilled by Carlos Fernandes in defending set pieces. During the game Amorim left the technical area to allow his assistant manager to oversee these. Yet, hilariously, Dean Huijsen’s opener derived from a free‑kick at which he was left unmarked, the defender’s goal kickstarting the Cherries to a repeat of the 3-0 win at Old Trafford last season.

Related: Amorim says Manchester United were nervous after rout by Bournemouth

Amorim has cited how the “storm will come” during his nascent tenure. Well, after his baffling selection at Tottenham that gave up the initiative by resting key players and ended in Carabao Cup knockout, a squall, at least, may be impending following this dire performance.

One popular argument is that the 39-year-old does not yet have the squad for his 3-4-3 to function slickly but if the basics fail, as they did here, it points to poor coaching from Amorim and his staff. Bournemouth, simply, convinced more in both areas and are now up to fifth place in the table. When Tyrell Malacia padded along the byline, to the right of Kepa Arrizabalaga’s goal, the defender tied his feet up and Andoni Iraola’s men cleared.

The Spaniard’s first save was a regulation clutch of an optimistic Bruno Fernandes effort from 25 yards, an emblem of United’s flatness. The captain and Amad Diallo were the twin No 10s here and the latter roved about as much as the former likes to, cutting paths left from his designated right-sided zone.

Joshua Zirkzee was a No 9 starved of a chance to pull the trigger. Manuel Ugarte launched the ball towards him, bearings were lost, and it pinged off a shoulder, the wind perhaps meddling with the flight.

There was no excuse for the Bournemouth opener. Ryan Christie’s floated free-kick from the right dropped on to Huijsen’s head and he flicked beyond André Onana. Zirkzee, the designated marker, watched on criminally, as indicated by his head coach’s disgust.

This made it a sixth game in a row in which United had conceded first. A second nearly followed when Evanilson let fly from the edge of the area: Onana, diving, repelled but pushed the ball out straight, not sideways, and Bournemouth rued having no one on the follow-up.

United’s plague of a quality deficit continued. Diallo teed up his captain and Fernandes, near the penalty spot, scuffed wide. Moments later Kobbie Mainoo released the same player and Arrizabalaga palmed away a snapshot. At a Diallo corner on the right Huijsen’s header cleared in a thicket of bodies.

Amorim, needing to shake up his side, replaced Malacia with Leny Yoro for the second half. But the status quo remained. Zirkzee shirked a 50‑50 with Milos Kerkez that had Amorim spinning in frustration. Then Onana’s indecisiveness caused a scare that featured Evanilson pilfering the ball and turning in a cross those in red scrambled to stymie.

Amorim acted again. Alejandro Garnacho and Rasmus Højlund came on for Ugarte and Zirkzee, Fernandes shifting back to operate with Mainoo in United’s engine room. The Portuguese hoped this would awake his stuttering side.

Instead, it backfired. In United’s area Justin Kluivert ghosted past Noussair Mazraoui; the defender, sluggish, scythed the Dutchman down, and Craig Pawson had an easy decision despite the inevitable protests to the referee.

Kluivert converted the penalty and United plummeted further. Amorim has bewailed his charges ceding the ball and the third goal derived from this.

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Mainoo was the culprit, near halfway. Suddenly those in white swept the ball left to Evanilson who fed Dango Ouattara. His pull-back was precise and Antoine Semenyo rammed home.

Garnacho summed up United’s dismal offering by racing clear and dribbling a weak shot. Marcus Rashford, despite being excluded again from the match-day squad due to “selection” – Matthijs de Ligt was also absent due to illness – was reportedly at the ground, and would have backed himself doing better.

As he has to, Lisandro Martínez struck an optimistic note. “It’s football and we have to accept it,” the defender said. “We are 100% confident that we will get success in this club. It’s a process, I don’t want to point the finger at one player. We win together, we lose together. It is a big pressure but we love that. I believe a lot in this group.

“We know how big it is at this club. The expectation is very high, it’s a process and you can see we are playing so well but we also concede goals and we have to work on that. I know the fans are impatient with us. We are the only ones who can change it.”

At the close the PA announcer wished a merry Christmas to all. For Amorim it will not feel too cheery at the moment, so he will hope for victory at Wolves on Boxing Day.