Ronaldo, who became the joint all-time Champions League appearance holder,
on his 177th appearance in the competition, fired United ahead after an exquisite pass from fellow Portuguese Bruno Fernandes.
United played for almost an hour with 10 men after Aaron Wan-Bissaka’s dangerous challenge,
on Christopher Martins earned the defender a straight red card.
They looked set to hang on for a point in an entertaining match.
Cameroon winger Moumi Ngamaleu’s equaliser.
But, a 96th-minute mistake by substitute Jesse Lingard allowed Jordan Siebatcheu to score the winner.
The home fans celebrated wildly while Ole Gunnar Solskjaer was left to reflect on what might have been.
Having missed out on qualifying for the knockout stages of the competition last season.
United now host Villarreal in their next Group F game on 29 September in a repeat of last season’s Europa League final.