Fernandes kept pushing United as they won 4-3 in extra-time with Amad Diallo scoring on 120 minutes.
He said: "We have a young squad, they need time and when they have the chance to play they have to show they have the quality to play for Manchester United. We always have belief, the problem is we have not been consistent with our performances.
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"We give 100% we always give everything but at this club we always have to have high standards. It's a part of the game to be criticised, that's why you play for Manchester United.
"This season has not been at the level we want but we want to improve. We have always said our goal was to get into the top four and try to win the FA Cup, we will do all we can for those goals. The FA Cup is a big trophy for this club."