The South African great had a spell at Carrington as part of former United manager Erik ten Hag's staff.
McCarthy recalled to Creamer TV: "I went to Man. United knowing that perhaps it was outside my depth, because I had coached Cape Town City, AmaZulu, and there they were asking me to coach top players at Man United.
“Was I just going to learn? That wasn't my mindset or my state of mind... 'They asked me to come and do a job. I'll show them. Yes, they may know many things, but they don't know everything, so I could teach them some things they didn't know.
"That was my mindset when I went there, I'm not there to learn, I'm also there to make a change and make a difference.
“To improve the situation and learn as much as I can from everything I'm giving there, and then also take something from everyone else."
McCarthy added, “Even Cristiano Ronaldo, one of the best players in the world, could still teach him something he didn't know, because I've been on this earth longer than him and I've done it before him, so there was something he could learn from me."