The wantaway superstar made his first appearance under new manager Erik ten Hag in a friendly against Rayo Vallecano at Old Trafford.
After being substituted at half-time, Ronaldo left before the end of the game, snubbing Ten Hag's post-match team talk.
The 37-year-old's early exit also meant he did not join his teammates in applauding the United fans, who had given him a warm reception despite his attempts to leave the club this summer.
Ronaldo has been pushing to leave the Red Devils in pursuit of Champions League football this season, but the club remain reluctant to sell.
It is understood the forward's agent has informed United of an offer, possibly from boyhood club Sporting Lisbon, and wants the Red Devils to name their price.
The Portugal sensation had vowed to remain professional while his future gets sorted, but United were left disappointed by his antics on Sunday.
A club insider told The Sun: "It is not a good look to leave early when you have been involved in the squad.
"It is different if you are up in the stands and not involved.
"You would expect someone who has played in the game to be in the changing rooms at the end to listen to the manager's thoughts.
"Also a lot of people were there to see him and he was not even involved in the clapping of the fans after the final whistle."