Fletcher is suffering from the bowel complaint ulcerative colitis and hasn't played a game since November.
Old Trafford boss Alex Ferguson hinted last week that his midfield maestro may struggle to come back from the illness.
But Norwich star Martin was diagnosed with the same problem three years ago and roared back to help the Canaries blaze a trail from League One to the Premier League.
And he told the Sunday Mail: "I hope Darren can find a way through this and play at his old level.
"When I got my diagnosis the specialist was brutally honest and said: 'We need to get this sorted because if you have this for a long period it will cause other things.'
"It's related to arthritis and you can get arthritic swelling around the joints which is no use for a footballer.
"You also get chronic fatigue. When I came home from training all I wanted to do was sleep. I was quite worried for a while but I took my medication and managed to avoid flare-ups.
"I did a lot of research on-line, through books and also through Crohn's and Colitis UK, who have been brilliant.
"I found a way to manage it but it was scary and unfortunately Darren's going through that at the minute. It's such a hard thing to deal with as you get told conflicting things from different specialists.
"There's no cure so you just have to treat and manage it. That's the hardest part - to find the way that works for you."