The 42-year-old has announced he will be leaving Old Trafford after 29 years at the club.
During a glittering playing career, he made more than 1,000 first-team appearances in all competitions for the European giants.
And Ferguson believes the Welshman’s remarkable career could be a one off.
He said: “People can judge footballers on many aspects of their game, but to play for Manchester United means there is a competitiveness and intensity in every single game.
“At our club you don’t play friendly games or easy games – each one is a cup final for the team we’re playing against.
“So when you consider that Ryan had to run up and down that left touchline for over 20 years in matches of that intensity, he is truly unique.
“What he managed to do was completely unheard of.
“He became a hugely important figure within the dressing room.
“All the younger players would go to him because he commanded such respect and had such great influence on his team-mates.
“I know that when I used to get Ryan, Scholesy, Gary and Rio Ferdinand in the office and chat to them about clamping down on the dressing room and making sure that everything in there was right, Ryan was the one among them who would be the foremost figure, no doubt about that.
“His place in the history of Manchester United and football is already assured.
“I doubt we’ll ever see a career like his again.”