The Portugal international will face his former club in a Champions League group stage double-header, beginning with the October 23 showdown at the Theatre of Dreams.
Ronaldo is still fondly remembered by United fans, almost a decade after leaving for Real Madrid, but Mourinho has warned that the five-time Ballon d'Or winner is out to down the Red Devils, just like he was when helping Los Blancos to a 3-2 aggregate win on his most recent meeting with United.
"I think he likes to come [back]," Mourinho told the club's official website. "My feeling when we played with Real Madrid here was that he was happy to come back and was happy to play at Old Trafford, but he was also happy to score and to win.
"He comes with a Juventus shirt, he went to Juventus to win the Champions League, he comes to Old Trafford to score goals and to win the match, but Old Trafford likes him and Old Trafford will always react in a positive way to him, which is a great thing."
Ronaldo has yet to get off the mark for Juventus in his two Serie A outings since arriving from Madrid in a £100m deal.