"I think we're talking around December for Vidic, maybe around Christmas time," Ferguson told MUTV
The 30-year-old defender missed half of last season with a cruciate ligament injury, but after playing in United's first four Premier League matches of the current campaign, suffered a new injury to the same knee that required surgery in September.
The Serbia international was initially expected to be out until late November, but that has been extended another month.
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