Our scouts and coaching staff have done a poor job assessing him as a player. He was at his best for Shaktar in their half of the pitch, pressing, intercepting and tackling the opposition players and then quickly giving the ball to the attackers. A modern, mobile defensive midfielder like Kante or Herrera in 16/17. This is why Guardiola wanted him at City to replace Fernandinho.
At United he has been asked to contribute a lot more in the opposition half, he literally started as a #10 in his last game. He's not suited to this role and will almost certainly flop at United should we expect him to contribute as an attacking player.