The 37-year-old was left on the bench for the opening day defeat to Brighton, even though Christian Eriksen was used in a false nine role.
Manager Erik ten Hag said it was due to a lack of fitness and he made a second half cameo, with the Red Devils looking much better with him in the side.
It has been a rocky summer in the relationship between Ronaldo and Man United this summer after the five-time Ballon d'Or winner informed them he wanted to leave.
Their failure to qualify for the Champions League was at the crux of the issue, but he has been unable to find a way out of Old Trafford.
He missed their pre-season tour of Thailand and Australia, for family reasons, before returning for showdown talks.
Ronaldo played just 45 minutes of their friendly with Rayo Vallecano, and left at half-time, perhaps indicating why Ten Hag felt he was not ready.
Now the Portugal superstar posted a picture on Twitter showing himself in training with the caption: "Hard work always pays off."
Ten Hag may be preparing to play Ronaldo in the clash at Brentford on Saturday, at 5.30pm.
That could also depend on the status of Anthony Martial, who likely would have started against Brighton had it not been for a late injury.
Martial performed well in pre-season and could be in contention to start if he has recovered from the injury he picked up.