There had been speculation that the Red Devils were keen to re-sign the Portugal international, who left the club in 2009 for a record £80m, in a reported new-look wage package.
"Signing Ronaldo has never even been mentioned in any of our board meetings," ESPN quotes Charlton as saying. "But he did go beyond being just a great player for us.
"He was just the right sort of person Manchester United had been looking for. But I once saw Real Madrid trying to get a player from another club, and I didn't think very highly of it.
"I didn't think it was right then and I still don't now. Cristiano is a great player, who scores phenomenal goals and he loves the club - but he is not our player."
Yesterday, the Spanish giants attempted to quash rumours that Ronaldo could leave the club at the end of the season by claiming that the club are "very happy" with the player.
The 28-year-old helped knock his former club out of the Champions League by scoring the winner in the round-of-16 tie on United's home turf last week.