A day after the club appointed John Murtough as football director and Darren Fletcher as technical director in part to help with recruitment at Old Trafford, the 18-year-old stepped off the bench to score an outrageous goal as United drew 1-1 with AC Milan in the Europa League.
It was just Diallo's third appearance after arriving from Atalanta in January but the confidence with which he flicked the ball over Milan goalkeeper Gianluigi Donnarumma with his head suggested he is quite at home at this level.
Ole Gunnar Solskjaer has been easing him in gently but it may not be possible for much longer. A Premier League debut cannot be too far away, particularly if the ankle injury which ruled out Marcus Rashford does not improve soon. Anthony Martial has also been added to the injury list after limping off here with a hip problem.
It was a shame for Diallo that his big night was spoiled by Simon Kjaer's equaliser in stoppage time but Solskjaer had to accept afterwards it was deserved. "We were a long way off," he said.
"Possession wise it was 50-50 but we were a bit too slow with the possession. It's sometimes hard on the back of a game like we had [against Manchester City] but that's the learning that we have to get into ourselves and we have to perform every single week because we are playing against good teams and these are definitely a good team.
"Very disappointed to concede with one of the last kicks of the game but probably a draw is a fair result."
Solskjaer wasn't banking on needing Diallo at half-time but United were so poor in the first 45 minutes, it was no surprise to see the teenager warming up at the break.
United's first half was summed up by Harry Maguire. First, he was hit in the head by Alex Telles' goal-bound free kick and then a minute later somehow managed to hit the post from six inches when it was easier to turn Telles' corner into the net at the back post. The look of disbelief on Maguire's face was proof of exactly how bad a miss it was but it could also have been the look on Solskjaer's face as he stood in the dressing room at half-time trying to explain a first half which Milan dominated.
Franck Kessie had the ball in the net twice in the opening 10 minutes only for the first to be disallowed by the linesman's flag, and VAR to rule he had handled in the build-up to the second. United were fortunate to go in at the break level.
Diallo's moment of magic looked to have won it before Kjaer's header in the second minute of injury time. If United want to win this competition, they have work to do in the San Siro next week. Diallo, who could end up costing £37 million if a series of bonus clauses are met, has already made a case to start that game.
Even with Bruno Fernandes, Mason Greenwood, and Martial all starting, United struggled to create many clear chances and it was Milan who ended the game having had more shots and shots on target, even without strikers Zlatan Ibrahimovic and Mario Mandzukic. For Solskjaer, there were few other highlights apart from Diallo's encouraging cameo.
"He is creative and he plays with freedom," said Solskjaer.
"It was a great pass from Bruno, that is what makes the goal. Amad has still got loads to learn but he will be better off after the goal."
After conceding so late, United are left banking on their fine away form to bail them out of trouble in Italy. Another moment of inspiration from an unexpected source would help.
"It makes It harder but we have to go and score goals and win the game and that probably will suit us," said Solskjaer.
"We are a team that's better when we go forward. Of course, we fancy our chances, We've been exciting and efficient away from home, particularly against [Real] Sociedad [in the last round].
"That's a template on how we want to play. We have to score one or two but we fancy our chances."