Ronaldo 37, 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.