The Anfield boss had intended resting his skipper for tonight's fourth-round tie, but Sissoko - who had been ill ahead of Sunday's draw with Arsenal - had not recovered sufficiently to play against the Coca-Cola Championship side.
Benitez said: "I would have wanted really to leave Steven out of the match. But we have too many injuries and Momo was sick again, so he had to play.
"And in the end he was the one who scored the vital goal. Cardiff worked very hard and were always dangerous from set-pieces, and that is where their equaliser came from.
"But then Gerrard got through to score a typical goal. His form is coming back, as are his goals. That is three now in successive games."
Cardiff boss Dave Jones, a former Everton player who has a son in Liverpool's academy system, admitted it was only Gerrard's intervention that stopped his side producing a more memorable result.
Jones said: "It was Gerrard that really killed us - only he could have turned the game like that. So I'm going to have to get my son to duff him up in training!"
Liverpool took the lead through French youngster Nabil El Zhar with a stunning 20 yarder in the opening minutes of the second-half.
Darren Purse headed Cardiff level, but Gerrard stormed through to grab the winner while the Welsh hordes were still celebrating.
Benitez had walked onto the pitch at the end to embrace Robbie Fowler while the Kop roared the name of a man who is still there idol.
Benitez said: "I just wished him well - he was great for us when he came back to the club. He has left us all with great memories of a great player."