They were in fifth, level on points with Chelsea, on course for a return to the Champions League, if scarcely playing like a team who belonged among the European elite.
And then they contrived to lose a lead at Old Trafford to a team in the relegation zone. Soungoutou Magassa’s equaliser was no less than West Ham deserved.
A goal from a defensive midfielder had the travelling Hammers singing the name of a celebrated predecessor, Billy Bonds.
As for United, this was further, unwanted evidence that Amorim’s team remain distinctly underwhelming. They have missed opportunities to be rather higher in the standings. It is their own fault they are now eighth and boos greeted the final whistle.
A team who had started to forge a reputation as set-piece specialists cost themselves victory by failing to defend a dead-ball situation. Nuno Espirito Santo, who conjured a response to the tame defeat to Liverpool, merits credit for his part, too.
The West Ham substitute Andy Irving had barely come on when he whipped in a corner. United allowed the diminutive Jarrod Bowen to meet it and his header required a terrific goal-line clearance by Noussair Mazraoui. It preserved parity for about a second as the summer signing Magassa followed up to score his first goal for West Ham.
But the changes two Portuguese managers made played their part. Amorim had tried to go on the defensive, taking off two of his front three. It did not work.
United conceded shortly afterwards and were left with too few attackers to then get a winner. Nor was it Amorim’s only questionable choice.
Without the injured Matthijs de Ligt, Ayden Heaven made his first league start of the season at the heart of a new-look back three. The teenager collected an early caution for ploughing through Bowen, cut a nervous figure and was replaced by Leny Yoro at half-time in what felt an admission by his manager that he was wrong to drop the Frenchman.
If United defenders had a prominence, it turned into a tale of three United right-backs, none of whom actually occupied a role Amorim has abolished at Old Trafford.
Mazraoui, pressed into service in the middle, had tried valiantly to preserve the lead Diogo Dalot, rebranded as a wing-back and switched to the left, had given United.
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Manchester United’s Casemiro in action against West Ham United’s Jean-Clair Todibo during the drawn Premier League match at Old Trafford
Meanwhile, a player who cost United £50m had a brilliant game. Sadly for them, that player was Aaron Wan-Bissaka, who they sold for £15m.
He marked his return to Old Trafford by clearing off the line to deny Joshua Zirkzee, who had met Amad Diallo’s cross with his thigh. Seconds later, Bruno Fernandes flicked the outside of the post with a half-volley. Wan-Bissaka also made a brilliant slide challenge on Matheus Cunha in his own box; his name was sung by the visiting Hammers, perhaps more than it ever was at Old Trafford in his time with United.
Dalot, meanwhile, has become a scapegoat for sections of the United support. A goal may restore a little of his popularity. When Casemiro’s shot was deflected, the left wing-back was lingering with intent by the penalty spot. He swivelled to finish, his shot just evading Wan-Bissaka.
It was a belated first Premier League goal at Old Trafford, seven years after joining, and United have needed more end product from their wing-backs, with the notable exception of Amad.
A problem for United was that only Fernandes, Amad and Bryan Mbeumo supplied much by the way of urgency and Alphonse Areola had to tip over the Cameroonian’s curling shot. It followed a well-worked, and long-range, one-two with Fernandes from a corner.
Zirkzee was ineffective; after a first league goal in 364 days on Sunday, maybe it was too soon for another. Cunha accomplished little on his return to the team after two games out.
United began sluggishly, as they often do, allowing West Ham to settle into the game. The concern for the visitors was that, missing the banned Lucas Paqueta, they seemed to have too little incision or invention. They had hinted at a menace on the counter-attack. Magassa offered a glimpse of what was to come by fizzing a shot into the side-netting.
Yet Senne Lammens was largely untroubled for much of the match. While United had raised the tempo after 25 minutes, West Ham responded in the last 10. And then United creaked and cracked.
Fifth place, and the Champions League, looked a long way away again.