Ousmane Dembele scored the goal of the game as Thomas Tuchel outsmarted Mourinho to hand the Portuguese his first defeat since taking charge of UnitedJose Mourinho's second game in charge of Manchester United brought his first defeat as his team lost 4-1 to lively Dortmund side in Shanghai.
Gonzalo Castro scored twice for BVB while Pierre-Emerick Aubameyang also found the net before the break. Mourinho made changes at - and after - the interval but the substitutions did little to alter the course of the game.
Ousmane Dembele produced the moment of the match to score Dortmund's third before Henrikh Mkhitaryan added a consolation for United against his former employers.
Aubameyang played a part in the opener when his free-kick dipped over the wall and was parried by Sam Johnstone. Dembele's follow-up was blocked but United couldn't keep out Castro's effort, which sailed just over a jumping Luke Shaw.
United offered little in response to going a goal down and were soon even further behind. Captain for the day Antonio Valencia was deemed to have handballed in the box and Aubameyang sent Johnstone the wrong way from the spot.
Dembele then offered a stunning reminder of why he's rated as the best young attacking talent in Europe, dancing past Shaw and Marcos Rojo before beating Sergio Romero at his near post.
Mkhitaryan's goal rattled Dortmund and threatened to start a comeback, but the change in momentum was short-lived and Castro scored the fourth for Thomas Tuchel's side to seal the win.
Here are six talking points from the match...
1. Depay fails to impress in striking auditionThe question of where Memphis Depay fits into this team is one that was left unanswered by Louis van Gaal, and Mourinho gave him the opportunity to play as a lone striker today.
If this was an audition, however, it was one that he failed. There were a few deft touches and some positive intent, but ultimately he was hauled off at half time without even a glimpse of a chance.
2. Aubameyang would thrive in EnglandOK, we didn't learn this today, but among all the talk of a dearth of strikers in Europe Aubameyang provided a reminder of what a club like Arsenal could have if they loosened their purse strings. At 27, the Gabonese striker is approaching his prime and it is believed a bid of £65million would be enough to secure his signature. That's similar to what Real Madrid want for Alvaro Morata and considerably less than Napoli are demanding for Gonzalo Higuain.
Aubameyang had a hand in Dortmund's first two goals and was a threat throughout. There may not be many strikers bristling with talent on the continent but this man is certainly one of them.
With transfer fees inflating every year, he looks like being worth every penny of his considerable price tag.
3. United light at the back?Mourinho said yesterday his transfer business for the summer was "75 per cent done". With three players already signed that makes Paul Pogba the final piece of the puzzle, but will that be enough?
Eric Bailly may have arrived at Old Trafford with a world class price tag but he's not quite the distinguished centre-half fans wanted.
And with Matteo Darmian still recovering from Euro 2016, Mourinho started Antonio Valencia at right-back. For a club willing spend close to £200million in a single transfer window can they really not do better than an ageing winger who spent half of last season on the sidelines?
4. Dembele's reputation growing by the dayIt's testament to the quality of Dembele that in a squad as talented as Dortmund's he is still the player that evokes the most excitment.
United kept him quiet for the best part of an hour but he still left his mark on the match.
The teenager picked up the ball 40 yards from goal, beat Shaw and then left Rojo on his backside before flashing a vicious shot past an immobile Romero.
5. Johnstone has a day to forgetSam Johnstone won't look back on this match with any fondness. The warning signs were there early on when his not-so-fancy footwork almost landed him in trouble.
The 23-year-old was came close to being dispossessed in his area by Aubameyang after a heavy touch, and he didn't look comfortable with the ball in his feet for the rest of the game.
He didn't fare much better with his hands either. The first real test of the game came from a free-kick. Aubameyang struck it from 30 yards out and Johnstone parried it straight back into the box. A scrambled ensued before Castro scooped the ball into the back of the net.
Aubameyang then stroked a penalty past him before he was hauled off at the break.
6. Mata proves his worth to MourinhoMourinho insisted on Thursday that he never wanted Mata to leave Chelsea when he was in charge and that it was the Spaniard who forced the move.
Whether you believe that line or not, Mata certainly has a lot to offer this United side and he showed it against Dortmund.
In an often stunted attacking line he was always available for the pass and capped his performance with an incredibly unselfish assist for Mkhitaryan against his former club.
The teamsManchester United: Johnstone (Romero 46); Shaw, Bailly, Jones (Rojo 46), Valencia (Pereira 75); Blind (McNair 66), Herrera, Lingard (Rashford 46), Mkhitaryan (Januzaj 66), Mata; Depay (Young 46).
Borussia Dortmund: Weidenfeller; Schmelzer (Burnic 63), Papastathopoulos (Bartra 62), Bender (Merino 62), Passlack (Larsen 46); Castro, Rode (Sahin 63), Dembele (Pulisic 63), Kagawa (Leitner 63), Ramos (Mor 63); Aubameyang (Hober 63).