Cech pulled off a stunning reflex stop to keep Hernandez at bay and ensure that Chelsea secured a 1-0 win over United in the quarter-final replay.
The Czech Republic international admitted that his performance against United gave him huge satisfaction as the Blues continue to chase two trophies and a top-four finish.
He told the club's official website: "It was a great ball from the wing and Javier did well to direct the ball into the goal. 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.
"I got a few messages from other players and keepers, it's great that people can appreciate the save. Even in the first half with the shot from Hernandez, when the ball moved away from me, it was a scary moment and it took a lot of energy for me to kick the ball away, so I'm pleased with the way I played."
The Blues are back in action tomorrow night in the first leg of their Europa League quarter-final against Rubin Kazan.