The Blues were a goal up when the Mexican connected with Danny Welbeck's cross with a firm header but the Czech Republic international somehow got a hand to the goal-bound effort.
It proved to be a vital save as Chelsea held on to book a semifinal against Manchester City at Wembley later in April.
"It was a great ball from the wing and Javier did well to direct the ball into the goal," Cech told reporters. "I went as fast as I could and left my left arm to do the job. I tried to cover as much space as I could and luckily I was able to reach it.
"I knew before the cross came that Hernandez was there, so I knew I'd have to be really quick to get there, that's what allowed me to make the save. You need to have awareness of the people around you as it helps you make the right decision. It was an important moment in the game, I'm sure a lot of people could already see the ball in the goal, so I was glad I got enough on it to push it over."