Cristiano Ronaldo had given United an early lead, poking home in the 13th minute off a wonderful outside-of-the-boot pass from Portugal teammate Bruno Fernandes, but things took a turn when Aaron Wan-Bissaka was sent off in the 35th minute.
Young Boys reacted to the red card and put some sustained pressure on the United goal. Their breakthrough came through Nicolas Ngamaleu after 66 minutes, the home side getting a little fortunate as a cross deflected off Luke Shaw. Fortune was theirs once again in the last minute when a stray pass from Jesse Lingard allowed Jordan Siebatcheu to nip in and find a winner.