Ferguson confirmed Vidic had a stomach infection and is unlikely to return against Chelsea.
"He was 24, 36 hours without eating," the United boss explained.
"He had some soup this afternoon, but wasn't ready to play. I don't think he'll be ready for Saturday, that's the situation, it's a stomach infection, (although) we don't know what kind of infection it is.
He added: "Yesterday before training, he had very severe stomach pains, so we sent him off to a hospital only about a mile away.
"We felt that was the best place for him to go. He was kept in until late at night and then they let him go."