Erik ten Hag's side were superior for most of the first half and led through Diogo Dalot's eye-catching goal, but a blink-and-you'll-miss-it double from Donyell Malen turned the scoreline and the mood at the awe-inspiring Allegiant Arena in Las Vegas.
Substitute Antony did restore parity shortly after the break, but Youssoufa Moukoko struck 20 minutes from time to make it 3-2 and leave United ruing several missed chances and defensive errors.
Here's your quick run-through of what happened in and around the game in Nevada...
ANOTHER MINDBLOWING ARENA
Opened in 2020, the arena where United and Dortmund met is truly astonishing.