The former Blackburn Rovers and West Ham United striker played under Mourinho at Porto over a decade ago and feels that the likes of Wayne Rooney, Anthony Martial and Marcus Rashford will only improve in the storied manager's care.
"People say he doesn't like young players, but I disagree with that. We had so many young players at Porto, youngsters who became great players.
"At Madrid, Di Maria, Isco, Jese and Raphael Varane were all young too."
McCarthy also suggests all of United's players will get so much out of Mourinho's management style, provided they carefully listen to instructions.
"No one should be fearful of him, unless they don't do as they're told and keep repeating mistakes," the South African added.
"If you're willing to learn under Mourinho, you'll do well.
"Footballers like waking up in the morning and looking forward to training with him."