Ronaldo notched his first Seria A goal in over a month on Saturday, tapping in a deflected shot to put Juventus up 1-0 on Genoa at Allianz Stadium in Torino, Italy.
The Argentine moved to Milan on loan in August after two seasons at Juventus, where he won two Serie A and Coppa Italia titles.
Tahith Chong trained with Manchester United's first team ahead of their Champions League clash with Juventus as a replacement for Alexis Sanchez.
With Genoa struggling to contain the lively Cancelo down the right Juve were firmly in control, their only regret at the break being they weren't further in front.
Napoli, coached by Carlo Ancelotti who will take his side to former club Paris Saint-Germain in the Champions League in midweek, were in front after 12 minutes on Saturday through a superb effort from Spanish player Fabian Ruiz.
Earlier, Roma came unstuck 2-0 to visiting minnows SPAL to dash their hopes of a fourth straight league win.
The Curacao-born 18-year-old is considered, alongside Angel Gomes, to be United's most exciting academy product and he impressed on the club's tour to the United States last summer.
Ronaldo's only outing against Mourinho's United was in the 2017 UEFA Super Cup, featuring for seven minutes as a substitute in Madrid's 2-1 win.