But Ferguson said his fellow Scot would “have a position at the club anyway” if he was unable to return to competitive football.
Fletcher has been sidelined since December after United confirmed the precise mystery “virus” that had affected him since the previous season.
And whilst Ferguson insisted United would provide Fletcher with “all the time he needs”, there must now be doubt over whether the Scotland international will ever play first team football again.
“He has not started training with us,” Ferguson said Thursday. “He has obviously got great challenges ahead.
“We are happy to be patient but he won’t be starting the season.
“It is unfortunate because he is such a magnificent professional and such a nice lad. If it doesn’t work for him, he knows he has a position at the club anyway.”
Ferguson was speaking at a news conference where he was introducing his two most recent signings, Japan star Shinji Kagawa, who has joined from German champions Borussia Dortmund, and England Under-18 international Nick Powell.
The veteran United manager said it was possible he could add at least one other player to his squad before the new season starts, with Ferguson’s side looking to regain the English Premier League title they lost to local rivals Manchester City on the final day of last term.
“There will possibly be one more (player),” said Ferguson. “We have always done well in terms of getting players in at the end of the season.
“That is the right time.
“But always in a European Championship or World Cup year, there is a delay in the transfer industry.
“That is over now and we are still trying to get one more in.”
Meanwhile Ferguson re-iterated his criticism of Paul Pogba, who left United for Juventus, saying the 19-year-old French midfielder had treated the club with a “lack of respect”.
“We were disappointed in Paul Pogba, there is no question about that,” Ferguson insisted. “He showed a lack of respect to the club. But he has made his decision now.
“Although we always felt he was going, he never indicated it until he left. That was disappointing.”