#1 United lost the midfield battle
Jose Mourinho's side started with 3 in the midfield and from the team sheet, it looked like they would dominate the two of Brighton. In reality though, Stephens and Propper nullified Pereira, Fred and Pogba as if there weren’t even playing!
United midfield was guilty of over-complicating things. Pereira, Fred, Pogba, all tried the odd tricks, the cheeky passes. They ignored the easy options and went for the extravagant. Perhaps there was a hint of overconfidence in their game. On the other hand, Stephens and Propper kept it simple. They pressed and pressed and kept the ball running. They were compact in the middle.
Pereira dropped deep to sit between Bailly and Lindelof and Brighton utilized the space to their advantage. The Brighton midfielders almost left Fred and Pogba behind them and used up the area between United’s defence and midfield.
Mourinho brought in Lingard in the second half, asking Pogba to play a deeper role. He then brought in Fellaini in place of Martial, who was poor, but neither of those moves worked. By winning the midfield battle and neutralizing Paul Pogba, Brighton won the game.