And the Sun say Mourinho ‘cannot stand’ Ronaldo - which may explain why he did not secure an emotional return to Old Trafford.
Together at Real, they helped the La Liga giants dethrone Barcelona as the division’s leading force.
But, ultimately, their relationship crumbled.
Guillem Balague reveals the details of their rift in his 2015 biography of Ronaldo. The two supremos nearly came to blows with Ronaldo screaming: "After everything I've done for you, this is how you treat me? How dare you say that to me!"
Man Utd news: Jose Mourinho and Cristiano Ronaldo at Real Madrid together (Image: GETTY)
Mourinho did not help the situation when he suggested Ronaldo had an attitude problem.
He said: "I had only one problem with him, very simple, very basic, which was when a coach criticises a player from a tactical viewpoint trying to improve what, in my view, could have been improved.
"At that moment, he didn't take it very well because maybe he thinks he knows everything and the coach cannot help him develop further."
And the Sun say the encounter between United and Juventus will be fascinating due to their fractured relationship.
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Man Utd news: Jose Mourinho and Cristiano Ronaldo in happier times (Image: GETTY)
Cristiano Ronaldo scores stunning goal in Juventus training
Ronaldo has played against United three times since leaving Old Trafford nine years ago.
In 2013, he scored in both legs as Real beat the Red Devils on away goals to reach the Champions League in the last-16.
And he came off the bench as Madrid beat United 2-1 to win the UEFA Super Cup.
Whether he can consign them to another European defeat, meanwhile, remains to be seen.