The Scotland international is training with the rest of the United squad after making his return to action against Aberdeen in a pre-season friendly earlier this week.
Last year Fletcher was diagnosed with ulcerative colitis, a chronic inflammatory bowel condition that some thought could jeopardise his playing career.
However, the 28-year-old is on his way to full recovery and Ferguson is pleased with his progress.
"Darren's fantastic, I mean obviously we are in the laps of the gods in terms of Darren because he has a certain condition that he has to handle very well," the Scotsman told MUTV on Friday.
"At the moment he is doing very well at that, he has put the weight back on which was a big concern for the kid.
"But he came on for 25 minutes in Aberdeen and did very well. He trained yesterday so he is fine."
Fletcher's appearance against Aberdeen was his first since November last year.