Bayern Munich prevailed, 4-3, after responding well to Man United's spirited comeback attempt in their UEFA Champions League Group A opener.
The German champion had a 2-0 halftime lead on goals by Leroy Sané and Serge Gnabry, and looked headed toward a comfortable victory until Rasmus Højlund struck in the 49th minute.
Harry Kane's converted penalty in 53rd minute swung the momentum back to Bayern Munich but it got a wakeup call with Casemiro's 88th minute strike, making it 3-2.
Five minutes after replacing Sane in the 87th minute, the 18-year-old Frenchman Mathys Tel scored a stoppage-time beauty to make it 4-2. Casemiro scored his second in the fifth minute off stoppage time, but the final whistle blew shortly after.