McCarthy joined United’s coaching staff at the start of ten Hag’s reign in 2022 and left after the FA Cup win last season.
The 47-year-old, who crossed paths with Ronaldo at United and starred at Porto under Mourinho, feels the opportunity was missed.
“You know what? It was brilliant working with Cristiano Ronaldo. It was brilliant because you've got a guy who knows football inside out,” McCarthy exclusively told Ladbrokes at the launch of Gaffer of all Accas.
”He became the greatest player of all time in my view – I don't speak for everyone else. So it was genuinely so nice to just see the drive and determination he still had at his age, you know, especially compared to the younger players around him.
“You know, if I were them, I would have smothered him with questions, asking this and asking that and wanting to know everything. His regime, his methods, his approach to staying as sharp and in shape as he is.
“What does he do? How does he live his life? I would have asked all those things. And it's a shame that our players never, ever latched on to him in that sense.”