Daniel James ploughed head-first into Jose Mourinho after a crunching tackle from Harry Winks, leaving both player and manager in pain.
Tottenham midfielder Harry Winks was booked for the tough tackle, which sent Manchester United winger James flying towards the advertising boards.
Spurs boss Mourinho was standing just outside of his technical area and James smashed into him with his face before landing in a heap on the ground.
Mourinho meanwhile could be seen hobbling out of the way, in obvious pain.
It was initially feared that James was in a bad way, but the Wales international eventually got back to his feet and resumed playing.
Marcus Rashford gave Manchester United an early lead at Old Trafford after beating Paulo Gazzaniga at his near post.
Ahead of Wednesday night's match - Mourinho's first return to Old Trafford since his December sacking - the Portuguese said: "At United I won and I learned. No regrets.
"I'm not a villain, I'm not an enemy but I am the coach that is trying to win against Manchester United.
"That's the way I think they [fans] are going to look at me. They're not going to support me, they're going to support [current boss Ole Gunnar] Solskjaer.
"That's the nature of the game and the nature of a place with an incredible history and fans who understand clearly the passion of the club. They love the club, they love Solskjaer.
"I want Tottenham to win, they want Manchester United to win. During the match I expect them to forget me and I expect them to support their team to get the result they want."
