Scotland midfielder Fletcher has fought back from the career-threatening illness that has sidelined him from first-team action since November last year.
He played two games for United reserves recently and manager Sir Alex Ferguson indicated last week he would be included in the club's Champions League plans.
Fletcher, who has been plagued by a chronic bowel condition, was due to train with Scotland this week ahead of their opening World Cup qualifiers.
Ferguson said last week: "Darren's put a lot of weight back on.
"He's back to his normal weight from two or three years ago. It's a case of wait and see."
He added: "But Darren comes into the 25-man squad quite easily, without me having to leave others out because we have a number of players who we don't need to register because of the time they've spent with us as an academy player.
"That's an advantage.
"He's training well every day and played really well the other night for the U21s."
Kolo Toure was left in Champions League limbo after being left out of Manchester City's squad for the group stage of the competition.
And the defender also failed to complete a move to Galatasaray in time to be named among their 25-man submission to UEFA.
Toure has reportedly been in talks with the Turkish giants about a move ahead of their country's transfer deadline on Wednesday night.
But the 31-year-old, who has just a year left on his City contract, could now see his value fall due to being ineligible for Europe's elite club competition.
Florent Malouda has been left out of Chelsea's squad, casting further doubt over his future.
Malouda was omitted from the 22-man selection to be submitted to UEFA ahead of the Blues' defence of their European crown.
The 32-year-old has not featured in any of Chelsea's matchday squads this season and has been heavily linked with a move away from Stamford Bridge. A transfer to Brazilian giants Santos reportedly collapsed due to the wage demands of the France winger, who has been at Chelsea for five years.
The Blues let Raul Meireles join Fenerbahce on Monday, while Michael Essien moved to Real Madrid on a season-long loan during Friday's transfer deadline day.
Veteran goalkeeper Hilario was also omitted from the squad announced today.