The Serbian defender was ruled out for two months on Tuesday after suffering more setbacks following his return from the same problem that kept him out for much of last season.
"I don't think he was ever 100%," said Ferguson. "I noticed a couple of times in training that he was limping a little bit. Then he started feeling tightness in his knee.
"He wanted to carry on and in that situation that is what tough guys can do.
"Obviously the main operation was to correct the cruciate but when you do cruciates there is always the danger that the cartilage or meniscus is damaged also and that was the case."
Ferguson defended United's medical team for not picking up on Vidic's limp earlier.
"The difficulty is for our medical staff to make a decision as to when players are ready to come back," he said.
"The worst that can happen is if you try and rush them back they get a training injury.
"That is unthinkable nowadays and that is what we are trying to avoid."
United are currently second in the Premier League table with 12 points from five matches.