The Czech Republic goalkeeper is confident that the Blues' ailing season will get the kick-start that it needs with victories over the Red Devils and in the Europa League
After a guileless defeat against Steaua Bucharest in the Europa League on Thursday night, the Blues' quest for silverware could be over if the Blues fail to navigate their cup tie against the runaway Premier League leaders.
However the 30-year-old Czech Republic international is confident that his team can retain the trophy, telling Chelsea TV: "We are still the holders of the FA Cup and we want to keep our trophy for as long as we can.
"The best way for us to forget about the defeat [against Steaua] is to get into another game. There will not be much time for us to prepare but hopefully we'll have a good performance on Sunday. The games are coming thick and fast but we are used to that.
"United are the favourites for the game but it's the FA Cup and in the FA Cup not every time do the favourites win. We have big players in our dressing room who are at their best in the big games. We have to show it again."
Despite their shock loss in Bucharest, Cech remains confident that Rafael Benitez's side will overturn the deficit in the home leg of their last-16 tie on March 14 and continue their Europa League campaign.
He continued: "We've turned [around] a much bigger deficit than 1-0 so I think this is nothing dramatic.
"You just need to make sure you score goals, you don't concede. We will be at home and it will be a different story.
"We had more chances in Romania, better chances, more shots. The only moment they really were dangerous was when they had the penalty and they converted it but we get a second chance and we have to take it."