Ten Hag fielded what looked like his strongest possible starting line-up for the first team this season, and United produced one of their best first halves to lead 2-0 at half-time.
Rashford returned to the side after being dropped for last Sunday’s FA Cup win at Newport County, and it took him minutes to open the scoring.
The 26-year-old combined well with Luke Shaw to create Rasmus Hojlund’s fourth goal in five games. And United should’ve entered the break with at least double their two-goal advantage.
A disputable penalty decision allowed Pablo Sarabia to peg one back from the spot on 71 minutes before Scott McTominay appeared to put the result out of doubt moments later.
However, it was an all-too-familiar sight as United collapsed late on. First, Max Kilman made it 3-2 with five minutes left before Pedro Neto looked to have netted an incredible 95th-minute equaliser.
Then, Mainoo, who has established himself as one of the first names on Ten Hag’s teamsheet, produced a moment of magic to restore the visitors’ lead in the seventh minute of nine added on.
The breathtaking moment cued pandemonium in the away end and dugout, and after full-time, Ten Hag heaped praise on the 18-year-old prodigy.
“It’s massive [the win]. This match had a lot of signs. But let’s first take the positives. We dictated the game for the first hour, and we should have been up 3-0 or 4-0,” Ten Hag explained to TNT Sports.
“At the end, we were naive in the way we conceded the goals, but we showed resilience, especially Kobbie Mainoo – it was a great goal.
“I’m not the manager telling him that I’m proud, but he’s making incredible progress. He has great abilities and he is a midfield player that can defend and attack. He has intelligence and also physicality.”