The Portugal forward missed United's pre-season tour to Thailand and Australia due to personal reasons as speculation swirled over whether he would be leaving Old Trafford.
It has been reported Ronaldo wants to play Champions League football, but United maintain the 37-year-old is not for sale and remains an integral part of the plans of new boss Erik ten Hag.
The former Real Madrid star played the full 90 minutes of United's 4-0 defeat at Brentford on Saturday, which left them bottom of the Premier League.
In a reply to a fan account on Instagram that referred to a report linking the United frontman to Atletico Madrid, Ronaldo said fans would "know the truth" in a couple of weeks, adding: "The media is telling lies."
"I have a notebook and in the last few months of the 100 news I made, only five were right. Imagine how it is. Stick with that tip."
Manchester United will seek to kickstart their Premier League campaign when they host fierce rivals Liverpool on Monday.
United's owners, the Glazer family, have come under fire following the team's terrible start.
Former United defender Gary Neville has warned the club could finish in the bottom half of the Premier League if they do not address their "horror story" recruitment strategy.
Manchester United supporters' group The 1958 is planning a protest march outside Old Trafford against the club's owners ahead of the Liverpool match.
In a statement the group said the ownership was "systematically starving and killing the greatest football institution in world football, for greed".
"We appreciate emotions are running high," it added. "We must keep it legal, we must keep it peaceful.... Bring the heat, bring the noise, bring the passion. Let's show the Glazer family that this time it will not blow over."
Billionaire Elon Musk sparked a social media frenzy on Tuesday evening when he raised the prospect of buying United from the Glazer family.
The Tesla CEO made the comment in a tweet, then later clarified his position by saying he had no intention of buying a sports team and that it had been a "long-running joke".
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