Ronaldo, who left United in 2009 to join Real Madrid before going on to play for Juventus, completed a move back to the Premier League club on Wednesday (AEST).
The switch had been announced last weekend, when United confirmed it had agreed a fee with Juve. The Serie A club subsequently confirmed United would pay €15 million ($24.2 million) for the 36-year-old in a series of instalments.
Ronaldo had seemed all set to join United's rival Manchester City, but with the assistance of club great Alex Ferguson – cited by the Portugal captain as the main reason for his Old Trafford comeback – United completed a sensational swoop.
However, after leaving Turin a week ago, Ronaldo had headed to Lisbon to complete a medical before linking up with Portugal's squad for its September internationals.
He went on to make history on Thursday (AEST), scoring two towering headers late on to seal a 2-1 comeback win over Republic of Ireland and, in the process, become the leading goalscorer of all time in men's international football, overtaking Iran great Ali Daei.
Portugal subsequently released Ronaldo, who has taken the number seven shirt he wore during his previous United stint, from its squad on Friday (AEST), as he had received a booking late on against Ireland and would have missed next week's FIFA World Cup qualifier against Azerbaijan.
Chronicling every major step in Ronaldo's homecoming, United detailed that the five-time Ballon d'Or winner had a day earlier touched down at Manchester Airport, where he was greeted by former team-mate and current Red Devils technical director Darren Fletcher.
"Cristiano is now preparing to begin training with Ole Gunnar Solskjaer's side at Carrington next week," United's account of Ronaldo's arrival read.
Ronaldo's second debut looks set to come against Newcastle United at Old Trafford on 12 September (AEST).