The Czech international acknowledges it has not been the best of seasons at Stamford Bridge but feels knocking United out of the cup can give the Blues impetus for this week's Europa League quarter-final first leg against Rubin Kazan and beyond.
"We are not having such a fantastic season, but this win [against Manchester United] shows we can still beat the top teams and it gives us confidence to play another game on Thursday," he told the club's official website.
"It's always important to beat big teams for the confidence. Any FA Cup progress is important for your season.
"We wanted to book a place at Wembley, we had great opposition and it was a great match. We are happy we managed to win the game.
"We seem to have a special relationship with the FA Cup. We treat it with respect, every season we try to win it and we get something back.
"We have another tough game against Manchester City, but it's a semi-final so you expect a tough game."