Fletcher fit for visit of Arsenal
The Scottish international has been suffering from virus which limited him to just two appearances between March and May last season and has so far kept him out of United's side this term.
After two reserve appearances, however, he is available to face Arsenal.
"Darren Fletcher played a game last night which is great news. I think the boy is back now," Ferguson told a news conference.
"He had a terrible time but he is now looking terrific. He will be in the squad for Sunday."
Ferguson also confirmed that Rio Ferdinand and Antonio Valencia were back in training but Nemanja Vidic would be absent for "another month".
Ferguson promised to continue with the youngsters who have brought a fresh dimension to United's play during the opening weeks of the campaign. Seven of the champions' starting lineup in Monday's 3-0 win over Tottenham were aged 23 and under.
The Scot said: "They have potential and we are very enthusiastic about that potential and hopefully they can fulfill it - the name of the game is what they achieve, today we've only played two games against West Brom and Tottenham.
"We've a long way to go, some of their form will taper off and then the squad will come into play, but at the moment they are doing very well and I have to let it carry on."
He added: "I am looking forward to Sunday, I think obviously a Man Utd-Arsenal game is always a contentious game and significant in terms of the rest of the season and that is why you always look forward to them."
Ferguson also took a swipe at the English Football Association when asked about the number of players his side were about to lose to England duty.
"The FA may realise who has produced more players for their country than any club in the world," said Ferguson, who received a five-match touchline ban last season.