Fletcher has been absent since December after United confirmed the precise mystery "virus" that had been affecting the midfielder since the previous campaign.
And whilst Ferguson insisted the Old Trafford outfit would provide Fletcher with "all the time he needs", there must now be some doubt over the Scotland star ever playing at the highest level again. The United boss confirmed: "He has not started training with us."
Ferguson added: "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 press conference to unveil his two summer signings, England Under-18 international Nick Powell and Shinji Kagawa, who has signed this summer from German double-winners Borussia Dortmund.
And the United manager is keen to add one more new face to his squad before the quest to regain the Premier League title from Manchester City begins next month.
"There will possibly be one more," he said. "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, in welcoming Powell from Crewe, Ferguson repeated his claim that Paul Pogba, who has left for Juventus, treated the Red Devils with a "lack of respect".
"We were disappointed in Paul Pogba, there is no question about that," said the Scot. "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."