Mccarthy believes that "worrying too much" is to blame for Marcus Rashford's poor form.
The 26-year-old came off at half-time during the Red Devils' 3-3 draw with FC Porto on Thursday, despite having scored and assisted already.
McCarthy spoke about Rashford's struggles with On The Whistle and stated: "He didn't have a worry in the world, now he worries too much about what's been said, what's been done, what's going on.
"If he can focus himself back to the Marcus that was just coming to training, loving being there, wanting to improve and work on all things - we used to work on every aspect of his game.
"As time has gone on, he's at a mental block where he's there but not playing to his full potential. He needs to get himself back to the place where he can block out all the noise, refocus and play like when he was in the academy, when everything you do comes from the heart.
"He has lost a little bit of that. When I watch United now, sometimes I don't see the player I used to work with. I just see a player that's not himself."