He was released from the Portugal squad after picking up a one-game suspension following their 2-1 victory over Ireland last week, and headed for England to isolate and then join training ahead of a potential second debut at home to Newcastle United on Saturday.
Ronaldo's arrival is sure to shake up the United starting line-up, and his teammates, including Fred, Aaron Wan-Bissaka, Edinson Cavani and Mason Greenwood were giving their all as they tuned up for the return of league action.
Manager Ole Gunnar Solskjaer has already warned his forwards that Ronaldo will not be warming the Old Trafford bench.
“He is not signing to sit on the bench. He is going to make us a better team,” said Solskjaer.
If Ronaldo is not deemed fit enough for an immediate return this weekend, he could play his first game in the Champions League when United visit Young Boys next week.
Image of the United players in training are in the gallery above. To move on to the next picture, click on the arrows, or if you're using a mobile device, simply swipe.