Manchester United suffered a stoppage-time 2-1 defeat away to Young Boys after a calamitous defensive mistake in their Champions League opener in Bern, Switzerland, on Tuesday, with United States forward Jordan Pefok netting the winner.

Cristiano Ronaldo's early goal put United 1-0 ahead before Aaron Wan-Bissaka picked up a 35th-minute red card in Bern on Tuesday.

A stunning curled pass from Bruno Fernandes allowed Ronaldo -- who equalled Iker Casillas' record Champions League appearances tally of 177 -- to tap home at the back post from close range on 13 minutes as the former Juventus forward scored his third goal in two matches since returning to United.

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A poor touch by Wan Bissaka then contributed to him going over the ball and heavily onto Young Boys' Christopher Martins. Referee Francois Letexier was quick to produce the red card.

United manager Ole Gunnar Solskjaer immediately took off Jadon Sancho for Diogo Dalot before Raphael Varane replaced Donny van de Beek at half-time.

Young Boys used their numerical advantage well and peppered United's goal in the second half, with the equaliser eventually arriving through Nicolas Ngamaleu on 66 minutes.

The Cameroon winger toe-poked his effort home from close range and almost added a second just a minute later before.

United thought they had earned a draw despite their second-half battering, but Pefok pounced on a loose backpass by Jesse Lingard to make it 2-1 in the fifth minute of stoppage time.

"In football in general, not just the Champions League, discipline is a huge part of the game," Solskjaer said in his postmatch news conference. "A lack of concentration, make a mistake and you get punished, you get done.

"Aaron is normally a very, very astute tackler, one of the best in the world, but it gave us a more difficult task. Jesse wants to play it safe and misplaces the pass, concedes the goal, that happens in football.

"Harry Hindsight is a very good player. If Jesse gets that chance again he turns, swivels and boots the ball the other way. You need 10 or 12 points [to qualify], win your home games, one away from home.

"We've lost the opportunity to get three points, but we've got two home games next and we've got to focus on those two."

Villarreal and Atalanta are the other sides in Group F.