Financial details of the transfer were not disclosed but British and Italian media reported United signed Ronaldo for 25 million euros.
Widespread reports in the Italian and English press say Ronaldo's agent, Jorge Mendes, has been finalising a transfer for the Portugal forward to English champions Manchester City.
The 23-year-old on Wednesday completed his move from Sassuolo.
Manolo Portanova of Serie A team Genoa has been placed under house arrest along with two other people on rape charges.
The 2-1 victory over Atalanta also handed a first trophy for coach Andrea Pirlo.
Kathryn Mayorga had accused the Portuguese football star of rape in 2009.
Spain's Atletico Madrid, Italy's Inter Milan, and six English teams abandoned the breakaway competition after intense criticism.
"True champions never break!" Ronaldo said on Instagram with a photo of him training ahead of Sunday's Serie A game away to Cagliari.
Borussia Dortmund striker Erling Haaland struck in each half as they advanced to the quarterfinals after a 2-2 draw with Sevilla in their last-16 second leg to complete a 5-4 aggregate win.
Borussia Dortmund's Erling Haaland continued his prolific run in the Champions League by scoring twice as his side came from behind to earn a 3-2 win at Sevilla in a last-16 first leg.