In a scathing interview, the ex-Porto and Blackburn forward accused the Dutchman of lacking passion.
Ten Hag is under a huge amount of pressure at United after a 3-0 loss to Tottenham.
McCarthy told Zero Zero: “Yes, it was inevitable. In the end he left the club and chose Al Nassr. He is proving to everyone that he can still score goals. If we work as a team and then let Cristiano do what he needs to do in his position, because he is the best in the world at it, that is ideal.
“I think Manchester United wasted a great opportunity to use Cristiano (Ronaldo) in the right way. But I was not the head coach and I could not be the one to make those decisions.
“It wasn’t easy because Cristiano is a huge character. He has his one personality, his own maturity, and he wants a coach who understands him too. At a certain point, it’s true, things started not working out between them.
“Erik had his philosophy, his ideas and he didn’t see Cristiano being part of them. And that’s when problems started. Cristiano wasn’t happy when he wasn’t playing because he gave everything he could in training. He trains like I’ve never seen anyone train before. He’s an elite player in every way, even in his behaviour.
“He would arrive at training very early, maybe two hours before everyone else started arriving. He was the most impressive athlete I have ever met in my life. Not all athletes have what it takes to be like that. Unfortunately, the coach’s philosophy was different from what he had envisioned.
“(Ten Hag) wanted a forward who could press up front. And Cristiano believed that this work should be done by the team as a whole. With him, in some areas, the best thing to do was to give him the ball and let him do his thing.
“Cristiano saw Ten Hag opting for (Anthony) Martial and (Marcus) Rashford in the ‘9’ position and he wasn’t happy because he felt superior to all of them. The coach saw things differently.”