The 28-year-old, who has been suffering with a chronic bowel condition, has resumed light training with the former Premier League champions and is expected to join up with the Scottish national side next week.
"Darren joined in for four or five days last week," Phelan told MUTV. "We decided that, if he felt well enough and felt good, we'd bring him into a training session.
"That's developed a little bit longer than what we actually thought. I don't know what the position is but if Scotland want to take him, it's probably just to be part of the squad and to introduce him back into that. But there's a long, long way yet for Darren Fletcher before we see him playing again."
Fletcher only managed 10 appearances for the Red Devils last season before he was ruled out in November.