Bobby Charlton: I'm delighted for "fantastic" Ferguson
Charlton was one of the United directors who appointed Ferguson in 1987.
The Red Devils boss has since gone on to win 13 Barclays Premier League titles - a feat that Charlton was quick to praise today.
"He is such a fantastic manager. Everything he has done has been fantastic," said Charlton at the launch for the Royal Mail's Football Heroes Special Stamps.
"He is a sensational person in every form and I am really delighted for him.
"I am a director but I hardly do anything because we are winning all the time and it is all down to Sir Alex Ferguson.
"He would get up in the middle of the night and travel 300 miles if he thought there was a school boy that he could sign.
"He loves the game and we at the club have had nothing to do really."
The former United striker thinks there were several reasons behind the 71-year-old's decision to call time on his career.
"He has made his decision on various grounds like (because of) his family and because of the hip operation he will have this summer."
Fellow United old boy Bryan Robson was another surprised by the decision of the man he will always call 'the boss'.
"It definitely came as a surprise because I go in to watch training every now and again," the former midfielder said.
"The boss hadn't mentioned anything, not that he would because he does not give too much away. Still, it is a bit of a surprise.
"It would have to happen at some stage and I think the boss must have just felt he has got a lot of grandkids and maybe he wanted to spend a bit more with them."
Robson, who retains close ties with United as a global ambassador, thinks Ferguson will get a say in who replaces him.
"The favourites are up there," Robson added.
"Mourinho gets mentioned all the time and David Moyes has been mentioned for quite a time.
"I think the boss gets on really well with both of them and I am sure the boss will have had a say in where the club do go next.
"The one thing I always believe with Manchester United, and that is why I am pleased to be involved with them, is that they do get a lot of decisions right and I am sure that they will get this one right."