United, who have been drawn in Group H with Braga, Galatasary and CFR Cluj, have to submit their 25-man squad for the competition to UEFA by Monday.
"Darren could come into the 25-man Champions League squad quite easily, without me having to leave others out because we have a number of players who qualify as youth products in addition to the 25," Ferguson told the club's official website.
"He's training well, every day. I think he's going up to Scotland again next week to work with them (the national squad). He played the other night for the U21s and played really well.
"With the condition he has, though, it's a case of wait and see. If he keeps doing as well as he's doing then obviously he'll be under consideration because he's a fantastic player."
Fletcher, who has ulcerative colitis, last played for United in November during a Champions League match against Benfica.
Ferguson also confirmed Wayne Rooney would be out of action for only four weeks after suffering a gash to his thigh in last weekend's win over Fulham.
"It's nothing serious on the actual gash - it's a bad one but it was treated carefully very quickly. We don't expect any complications from it. I think he'll be back in four weeks," Ferguson said.
"The fact it's a gash and not a muscle or joint injury means he can do a lot of work in the gymnasium."