Torres had barely trained in the fortnight since he picked up the injury in the 1-0 first-leg victory in the Bernabeu.
Manager Benitez said: "Fernando had only light training for three days but he desperately wanted to play against his former club.
"He caused so many problems for their defenders, they could not handle him or his passion. But there was great commitment from all of my players.
"Fernando did his best to convince me to let him play. We talked a lot about it beforehand and he kept telling me he was 100 per cent.
"But he still needed an injection in the ankle and it was heavily strapped up. Playing against Real, particularly with his past at Atletico Madrid, was so important for him. The strapping was very strong, but we felt it was right to take the gamble."