Ferguson does not believe Keane can recover from the cruciate ligament injury he suffered against Leeds earlier this month in time to play in France, should the Republic beat Belgium in the play-offs.
"I think we could have him back training in April, but there's a big difference between training and taking part in football," Ferguson said. "You're looking more to the start of next season rather than the start of the World Cup in June."
Although the Argentinian club River Plate have now expressed an interest in selling their Chilean international striker Marcelo Salas, Ferguson has denied that a move to United is imminent for Salas.
Blackburn Rovers are to allow their Danish striker Per Pedersen to join the German side Borussia Monchengladbach on loan for the season.
The Southampton striker Egil Ostenstad is to consult a specialist after breaking down with a recurrence of the ankle injury which has dogged him all season. The Norwegian, who failed to complete the first half of Tuesday's Coca-Cola Cup tie against Barnsley, could be out for several months.
Tottenham have denied reports that their striker Les Ferdinand has had a second operation on a stomach muscle injury and will be out of action for up to three months. He had a minor operation to cure an abdominal strain last Friday.Reuse content