The Serbian centre-back was stretchered from the field during United’s 2-1 loss to Basel on Wednesday night, a result that sent the Reds tumbling out of the Champions League.
Vidic left the stadium on crutches and his agent revealed on Thursday night that the defender had “suffered a ruptured cruciate ligament”.
Sir Alex, speaking at his pre-match press conference on Friday morning, confirmed what United fans feared most.
“We thought it was a bad one,” he said. “He’s out for the season. It’s not unexpected but it’s still bad news.
“He’ll see a specialist on Monday to work out the progress of when he has the operation. It won’t be immediate as we have to let the swelling come down.
"We'll now have to choose between Jonny Evans, Phil Jones and Chris Smalling [to fill the gap]. I thought Rio had his best game of season on Wednesday. His experience will be vital.
"[Losing Nemanja] is a loss and we missed him in the early part of the season for five weeks with his calf injury against West Brom.
"He's such a dominant character, particularly his defending in the penalty box. That's always a strong feature of his game."