United paid £35million for the 23-year-old midfielder this past summer.
He started last week's Carabao Cup win over Luton Town but was once again on the bench against Brighton on Saturday, and his agent, Sjaak Swart, was not happy.
"A substitute, I don't like it at all," Swart told VoetbalPrimeur.
"I couldn't do it myself, substituting in with four minutes left to play. Then you'd better let me sit down.
"I have to say he did three more good things. The penalty, where the winning goal came from, came from him.
"But normally they should have lost 1-7. Five times post and bar, Brighton … They have a nice team, but that shouldn't happen to Manchester United."