There is talk of United being prepared to sell Mainoo this summer.
But Murphy wrote for the Daily Mail: "I am surprised he has not featured more for United this season, especially with their problems in midfield, although I appreciate he has had injuries.
"I know (Ruben) Amorim thinks there has been weaknesses in his defensive work in certain games but that’s normal for a young midfielder in the Premier League who is adapting to playing different positions in United’s system.
"Amorim’s system has stifled him. Playing central midfield in a 3-4-3 is tough. You must be really disciplined and cover a hell of a lot of ground. It’s clear he doesn’t trust him as much as Casemiro or Manuel Ugarte. He has worked with Ugarte at Sporting Lisbon, so knows he understands what he wants from his midfielders.
"But I don’t see what Amorim sees when it comes to Mainoo. I think Mainoo is more than capable of playing that defensive role, he just needs some help, leadership and guidance around him."