Ruben Amorim’s wish to “create some danger” flamed at the start of each half, then faded, as Manchester United were sucker-punched by Jean-Philippe Mateta’s second goal on 89 minutes that sealed a first defeat in four games. United and Crystal Palace were becalmed in front of goal except for when the Eagles twice took advantage of the home side’s slipshod backline. Mateta’s second was simple. Ismaïla Sarr shrugged off a challenge near halfway and slid the ball into Daniel Muñoz. He ran forward and squared to the No 14, who did not miss.

This followed André Onana’s leaden feet causing the Palace opener: the home keeper failed to tip over Maxence Lacroix’s header of Eberechi Eze’s 64th-minute free-kick, the ball smacked off the bar and Mateta steered home. The desire for more attacking menace was behind Amorim’s refusal to select Rasmus Højlund or Joshua Zirkzee and deploy Kobbio Maino as a false No 9 – a first for the head coach.

It failed, though you can see the Portuguese’s reasoning, as Mainoo is a far better technician than Højlund or Zirkzee so gives United ball-stickability up front and, as the FA Cup winner illustrated, he has a nose for goal. In one fluid back-to-front move there was evidence. Bruno Fernandes lifted the ball to a galloping Nossair Mazraoui. He passed to Mainoo in the area, the No 37 glided into a wide right zone, twisted, and crossed: Alejandro Garnacho’s shot claimed a corner that yielded nothing.

Then Mainoo went close. His marksman’s eye was on show as his effort clipped Dean Henderson’s right post, Palace’s keeper left marooned. Next, too, was Onana when a bright Palace attack splayed the home rearguard. Diogo Dalot admired rather than challenged Tyrick Mitchell’s ball over from the left and Muñoz should have scored but headed wide. Amorim talked of wanting to “improve” how the opponent’s final third was entered but if they impressed, in flashes, in this department, there is a greater problem in their defensive one.

It’s a continual problem Amorim also needs to fix. Example: Daichi Kamada could slip in behind unguarded, be found, and turn and set up Mitchell. The wing-back’s effort flashed across Onana’s goal and Mateta was a slightly bigger forehead away from connecting and registering. As a marauding force United had dissipated. They awoke, briefly, to knit a few passes together along the left of the Palace area yet when the ball was rolled back to Lisandro Martínez he blazed high.

When Onana clutched a smartly worked Palace free-kick, he threw the ball to Mazraoui, who careered along the right. Amad Diallo took over, spotted the in-trotting Fernandes and the captain’s first-time hit missed marginally. Sign up to Football Daily. Kick off your evenings with the Guardian's take on the world of football. after newsletter promotion. There was more: this time at a United dead ball, Fernandes’s curved delivery was met by Harry Maguire and Ugarte, sliding, unloaded, but Henderson saved well.

This phase of this half resembled the opening of the first half: United pressed but lacked teeth. A sweeping Martínez ball was pinged on a left-right diagonal into Diallo but he failed to find goal. On the hour Oliver Glasner brought on his star man Eze, who was not fit enough to start, but healthy enough to create the opener. The winger’s free-kick was aimed at Lacroix, Lenny Yoro was seriously outjumped, the defender’s header beat the flailing Onana and pinballed off the bar for Mateta to drive home.

With 20 minutes to go Højlund replaced Mainoo, and Zirkzee was introduced for Mazraoui. For this game, at least, the Mainoo experiment had failed. Before this Eze’s ability to bodyswerve cast Martínez as a defender drafted in from United’s junior ranks and if the No 10 had not been dazzled, too, by his snake-hips – falling over as he tried the trick again – Palace’s one might have become two.

Literal injury was piled on top of the insult of Palace’s ascendancy when Martínez crumpled and needed to be carried off on a stretcher. It was reminiscent of a similar episode, against West Ham last February, when the Argentinian suffered medial collateral ligament damage and was out for nearly two months. The hope will be that the 27-year-old is not as seriously hurt. At the close Palace’s fans were jubilant and Amorim had more explaining to do.