While Tottenham look to be recovering from their injury crisis, United's continues to get worse.
This week, Amad Diallo, their top scorer in the league, was ruled out for a long spell with an ankle injury, while Kobbie Mainoo, Manuel Ugarte and Toby Collyer also picked up knocks on the training ground.
With Christian Eriksen and Leny Yoro both missing the match through illness, Amorim was forced to hand veteran midfielder Casemiro his first start since 30 December, and the 32-year-old was often exposed against Maddison, Son and Tel.
Meanwhile, all the players on United's bench, except 30-year-old Victor Lindelof, were teenagers who had not made a first-team appearance for the club.
Amorim did not make a substitution until the 90th minute when 17-year-old striker Chido Obi replaced Casemiro for his United debut.
Still, United had plenty of talent on the pitch and should have equalised when Bruno Fernandes played in Garnacho before the break only for the Argentine to blast well over the bar.
Garnacho had a one-on-one saved by Vicario in the second half, while Joshua Zirkzee headed Noussair Mazraoui's cross narrowly wide from a good position in the closing stages.
Defeat was United's 13th of the season in all competitions and their ninth since Amorim took charge in November.