Captain may return at Anfield but only if he is fully fit, while veteran manager says Anfield opponents are still the club's biggest rivals despite the emergence of Manchester City
Vidic played for Serbia during the international break but Sir Alex is cautious about bringing him in to the team in such an important and physically demanding game.
"The big question mark is whether Vidic is ready for the game tomorrow," he told reporters.
"He's had his first game for Serbia on Tuesday and played a full game, so it's certainly a consideration because he's such a good competitor.
"He came in yesterday and felt okay but the Liverpool game is such an intense, draining game that you want everybody to be 100 per cent so it maybe more down to how he feels himself really.
"Everybody else is fit, apart from Tom Cleverley and Rafael."Liverpool have failed to challenge for the title in recent years but Sir Alex still regards them above Premier League contenders such as Chelsea and even United's local rivals, Manchester City.
"Although things may change in the next few years with Manchester City, I'm not sure whether the derby game can be equal to Manchester United-Liverpool game," he explained.
"I've always considered it to be the game of the season in English football and at the moment I think it remains that.
"I think the fans play a part, there is no doubt about that.
"In my time we've had nine players sent off there which is unusual for Manchester United so that is all down to the atmosphere that can be created there."
Sir Alex also claimed that Kenny Dalglish's side could be title contenders this season in what he predicts will be an open race.
He continued: "Four or five teams will challenge for the title and you always have to include Liverpool in that."