The 37-year-old Portugal forward, who missed the club’s pre-season tour to Thailand and Australia for personal reasons, reportedly wants to leave the club he rejoined last year.
Former Real Madrid and Juventus star Ronaldo finished as United’s top scorer last season with 24 goals.
But the campaign as a whole was a huge disappointment for the club, with a sixth-place finish in the Premier League meaning they missed out on qualification for the Champions League.
Ronaldo was left out of United’s squad for Saturday’s friendly with Atletico Madrid in Oslo.
But, commenting on a post on a fan page about him missing the game in Norway, Ronaldo wrote Friday: “Domingo o rei joga” which translates as “Sunday the king plays”.
Ronaldo applauded fans on both sides of the ground as he led the team out for their pre-match warm-up on Sunday.
The veteran forward had one clear chance during his 45-minute appearance but drove over the bar after running on to Donny van de Beek’s lay-off.
– ‘Happy to be back’ –
After the match, Ronaldo posted a photograph of himself playing in the game on his Twitter feed beneath a caption of “Happy to be back”.
Erik ten Hag, United’s new manager, told the club’s website: “It was a mixture between experience and youth (today).
“You saw the attitude was really great, really the intention to co-operate. They fight and work for each other. That was positive.”
New signings Christian Eriksen and Lisandro Martinez both featured for an hour in a game set up by Ten Hag for the players who did not have major roles in Saturday’s 1-0 defeat by Atletico.
Amad Diallo, who was on loan at Rangers last season, gave United the lead early in the second half when the 20-year-old Ivory Coast winger fired in the rebound after a shot from Alex Telles had been saved.
United’s lead, however, lasted just nine minutes before Alvaro Garcia equalised in similar fashion after Tom Heaton had saved Isaac Palazon Camacho’s initial effort.
United start their Premier League campaign at home to Brighton on August 7.