The Scottish midfielder has taken an extended break from football after being diagnosed with ulcerative colitis.
The Manchester United manager also revealed that the 27-year-old has returned to the training ground, but only to visit rather than train. Fletcher has received support from Olympic rowing legend Sir Steve Redgrave, who was diagnosed with the same condition in 1992.
"I still wouldn't be able to put a time limit on it to be honest," said Ferguson.
"I think there's been a lot of support from different sections. There have been a lot of letters in from people who suffer from the same illness.
"I think the important thing is to trust the doctors looking after him because they're the ones that are going to be mapping out what to do with his future."
Fletcher has been at Old Trafford since 2001.