The Manchester United midfielder has suffered from ulcerative colitis for five years, and was forced to take a break from playing in November 2011.
A comeback was curtailed after five games one year later, and the 29-year-old had surgery in January 2013.
Ferguson mentioned Fletcher in his farewell speech at Old Trafford after his final home game in charge of United before retiring, a move that stunned the Scotland international, but also left him determined to return to playing once again.
"I was just blown away," admitted Fletcher in the Daily Record. "For him, at that moment, to take the time to think and to speak of me was fantastic and it just shows the measure of the man.
"It speaks volumes of him to think of other people when the whole occasion was about him.
"It was just one of those moments that shocks you because you just do not expect it. It was emotional. More than anything it gave me a great determination to come back.
"Him speaking like that - after everything he has done for you - you want to get back as a thank you almost to him and to the club for the way they have been with me."
Fletcher will miss the start of the season, including the England v Scotland international at Wembley on 14 August.
Meetings between the two countries have been rare since annual fixtures were abandoned after 1989, but Fletcher would relish the chance to see the clash resurrected on a regular basis.
"It is a massive one. That game was my target. I was desperate to get fit for it," Fletcher added.
"I am so envious of the lads playing. What a chance they have to put their name in the fans' hearts forever by winning at Wembley. Everyone would remember that.
"That is the opportunity they have and I really believe they can do it.
"Hopefully this can be seen as a trial run and if everything runs smoothly and it is a great occasion then it would be fantastic to have it every couple of years.
"I hope that is the case because I am desperate to play in a Scotland v England game."