The winger has suffered a setback in his recovery after picking up a knock in the 5-0 win over Fulham.
The United boss revealed that Young has a bleed on the injured knee, keeping him out for much longer than anticipated. The England international joins the long list of absentees for Manchester United and will now miss all of the January fixtures.
"Ashley is four weeks away easily," said Ferguson.
"The bleeding never came out after the Fulham game and is only starting to come out now. That has been unfortunate.
"We don't think it is a serious injury but we can't prejudge it until we have all the facts."
Young joined United from Aston Villa in the summer for around £18m.