Torres was still troubled by a groin injury that needed a pain-killing injection and Benitez, seeking to silence the home fans' boos after four consecutive defeats in all competitions, only decided to risk him a few hours before kickoff.
It proved an inspired move as the Spain striker held off Rio Ferdinand to put Liverpool ahead in the 65th minute before David Ngog added a second in stoppage time.
"It was a difficult decision on Torres because he has not been training," Benitez told reporters.
"You could see at times that he was not fresh but sometimes an 80 percent Fernando can make all the difference."
Benitez, under increasing pressure after league defeats by Chelsea and Sunderland and Champions League losses to Fiorentina and Olympique Lyon, said his players had answered their critics.
"The passion, attitude and character of my players was the difference," he said. "Everybody worked together from the beginning and the players answered a lot of questions today.
"We showed this team is better than people think. It was just a question of getting a little bit of luck, then the character would shine through.
"We needed to win for the confidence. I think it will be a big boost for the team and the fans. Hopefully, we can use this to kick-start our season."
MASSIVE WIN
Torres also stressed the importance of the victory.
"It's a massive win for us," he said. "We showed that when we play as a team and work hard, we can trouble anybody. Now we have to keep working together as a unit."
United manager Alex Ferguson was upset Liverpool defender Jamie Carragher was not sent off for bringing down Michael Owen as the substitute threatened to break clear late in the game.
"The controversial one that people will be talking about was when he (Carragher) brought down Michael Owen. It would have been difficult for them if he had gone off," he said.
Carragher disagreed, saying: "I thought he brought me down. I don't think I should have been sent off."
