The Portuguese winger is set to join Real Madrid for a staggering £80million, prompting a whirlwind of speculation over who will be replacing him.
Sir Bobby said: "I'll have to leave that (deciding how to replace Ronaldo) to Alex Ferguson.
"He is a great judge of a player, he's very brave with his decisions, he's very decisive with his decisions and that's why he's been a great success. Who the players will be I don't know, but they will be worthy of putting on the shirt for Manchester United."
Charlton concedes that there was little point in trying to coax Ronaldo away from his childhood dream of joining Real, although he feels that hindsight may show the winger that his best days of his career were at Old Trafford.
"He is a great player," he said. "He's been with us for five or six years and in that time he has improved beyond all recognition. I suppose, in time, he will reflect, and say that the best time he ever had in football was when he was at Manchester United. It was better for him and it was fantastic for the club.
"But we have no divine rights, we talked him out of it for one year but he's gone and we move on. Ronaldo leaves us because as a young boy he always dreamt about playing for Real Madrid, so if he leaves us, it's with our blessing.
"It's a lot of money, but it's a little bit soul-searching. You never know if you've made the right decision until the start of the season. It's an enormous amount. We will make sure we will use it properly and our club will be better for