Speaking to BBC Sport Africa, McCarthy said: "I was given an incredible opportunity to work with one of the biggest and best clubs in the world under the guidance of Erik ten Hag.
"I learned utmost professionalism because the manager I worked under was not just a disciplinarian, but he was the best and one of the most unbelievable professionals I've ever seen in the game.
"Extremely hard-working, very dedicated, very passionate, he planned on being the Manchester United coach for the next 10 years. That was his vision."