After the delirium of Cristiano Ronaldo's stunning return, came this brutal reality check for Manchester United.
Ole Gunnar Solskjaer may feel he has a "special" squad now, with Ronaldo, Raphael Varane and Jadon Sancho on board, but they are still capable of self-inflicted damage, as this defeat proved.
Aaron Wan-Bissaka was the first villain of the piece, his poor touch and reckless action to try to redeem it leading to a straight red card in the first-half, which ultimately cost United the game.
Then it was the turn of substitute Jesse Lingard to be cast in the role of culprit, his kamikaze back-pass in the 95th minute letting in Jordan Siebatcheu, who scored past David De Gea to secure a famous win.
Until Wan-Bissaka's dismissal, United were in charge of their Group F opener, courtesy of Ronaldo's third goal in only his second game since returning, set up by compatriot Bruno Fernandes.
But Wan-Bissaka's rush of blood to the head ultimately shifted the momentum in favour of Young Boys, who equalised and then seized the moment to claim victory in the final minute of added time.
Manchester United's Raphael Varane and David de Gea react after BSC Young Boys' Jordan Siebatcheu scored their second goal ( Image:
REUTERS)
The match statistics at full-time told the story of this one-sided contest, the spirited and ebullient Swiss champions producing 20 attempts on goal to United's meagre two.
For all the flair and attacking options at Ole Gunnar Solskjaer's disposal, that stat is a dismal return for United, who were once again found wanting in Europe.
Solskajer claimed his players had learned lessons from last season, when they failed to exit the group stage, then suffered a shock penalty shoot-out defeat to Villarreal in the Europa League final.
But United looked as naïve here as they were in Europe last season, and must wise up quickly if they are to make any kind of impression on this season's Champions League campaign.
A group that contains Young Boys, Atalanta and Villarreal should be navigated with ease by United, but they cannot afford many more off-nights like this if they are to reach the knockout stage.
As expected, Ronaldo started and led United's attack, just as he did on his emotional Old Trafford homecoming at the weekend, when he scored twice in a 4-1 win over Newcastle.
Ronaldo's 177th Champions League appearance equalled the record set by his former Real Madrid team-mate Iker Casillas, but this was not a night for basking in such personal accolades.
Young Boys, managed by former Huddersfield boss David Wagner, had come through three rounds of qualifying to reach the group stage and were fully deserving of their historic triumph.
They had to come from behind, Ronaldo turning the ball over the line in the 13 th minute from an outrageous pass from Bruno Fernandes, with the outside of his right foot.
But that impressive start evaporated with Wan-Bissaka's straight red card for a studs-up lunge on Christopher Martins Pereira, the United defender having initially missed the ball.
Things went wrong for Solskjaer's men after Wan-Bissaka's red card ( Image:
AFP via Getty Images)
Wan-Bissaka could have no complaints with the decision and from then on it was an uphill battle for United, one they were incapable of winning.
The pressure from the hosts eventually told in the 66th minute, Moumi Ngamaleu scoring at the near post to make it 1-1.
Solskajer took Ronaldo and Fernandes off in a double change with 18 minutes left, both players unimpressed with their early withdrawal.
That decision was compounded when Ronaldo's replacement, Lingard, handed Young Boys the winner right at the end with his crazy back-pass, which was intercepted by Siebatcheu, who swept the ball into the net to send the home fans wild and United into a painful inquest.