Amorim has come under fire this season for failing to trust his academy players.
On a night when home-grown pair Kobbie Mainoo and Shea Lacey remained on the bench, evidence for Amorim's caution was seen in the performance of 19-year-old defender Ayden Heaven.
Given his first start of the season at the centre of United's famed three-man defence, Heaven was turned by 33-year-old striker Callum Wilson, and fortunate not to be punished as the West Ham man failed to control with a clear run at goal ahead of him.
Less than 10 minutes were on the clock when Heaven rashly charged into Bowen and was booked.
It was part of a skittish opening from the home side and they did not settle until close to half-time, when Joshua Zirkzee's improvised effort was cleared off the line by former Manchester United defender Aaron Wan-Bissaka.
That was followed by a Bruno Fernandes shot which flicked off the outside of a post.
It was a game of few chances though and, against limited opposition, Dalot's first league goal at Old Trafford - and only his fourth in 151 league appearances - appeared to be enough.
But not for the first time in recent weeks, their limitations were exposed and they failed to see a game out from a winning position.
Just over a week ago, after his team were beaten at home by 10-man Everton, Amorim said United were a long way from where they need to be. This was further evidence he was speaking the truth.
West Ham analysis: Wan-Bissaka excels on Old Trafford return
It is well known that Manchester United looked at 804 right-backs before deciding to spend £50m on Wan-Bissaka in the summer of 2019.
The former Crystal Palace player was not terrible during his 190 appearances for them, but neither did he look like he was worth the money either.
Wan-Bissaka's inconsistencies have followed him to London Stadium, where he was converted into a wing-back during Graham Potter's time at West Ham, and following Nuno Espirito Santo's arrival he is now playing in defence again.
The Londoner enjoyed his two meetings with his old club last season, as West Ham won both. He also made a significant impact here as he headed Zirkzee's effort off the line to ensure the closest the home side came to a goal before the break did not actually end up being one.
Wan Bissaka's diligence might turn into a big asset in their scrap for survival, as could Magassa's eye for goal.
The £17m summer arrival from Monaco was making only his eighth Hammers appearance and started for just the fourth time.
But the way he latched on to Mazraoui's clearance and instantly drove it back beyond Senne Lammens - and then the way he celebrated - suggested he is determined to succeed.
It is an attitude that might be priceless in the months that lie ahead.
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