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 under-21s."
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