Abdallah Sima had the ball in the net in the 59th minute, only for referee Nick Walsh to rule it out for a foul on Hoops keeper Joe Hart.
The Dutch manager is under pressure after a poor league campaign but finished the season with a stunning FA Cup win over rivals Manchester City
Manchester City 1-2 Manchester United: First-half goals by Alejandro Garnacho and Kobbie Mainoo stunned the Premier League champions at Wembley