He said: "I have a responsibility for all my players and their futures are a big concern for a coach. If you were asking advice and it was your son, you'd give him the best advice possible on his career.
"He's 23 years old and he's at a fantastic football club. It's the best for him."
Ronaldo signed a long-term deal last season in which United won the Champions League and retained their Premier League title.
He is currently sidelined for three months after an ankle operation following the European Championships.
Ferguson reiterated that he expects Ronaldo to play a massive role again next season.
He said: "The future is he signed a five-year contract last year and he has four years left. That's definite. I know that, I've seen his signature.
"Young lads can be twisted by the talk of money and the moves that are happening all the time in modern-day football.
"After the final in Moscow he was the happiest boy in the world. I think he knows genuinely where the best club is to be and that's Manchester United."
