Jesse Lingard came off the bench to score United’s third goal, before Romelu Lukaku wrapped up the win in stoppage time.
Lingard said: “Our attitude was spot on, it was always going to be tough.
“We adapted well to the ground and all made an impact.
“The FA Cup is very important, it’s great to win trophies, that’s what we are here for.”
And Herrera, who scored his first goal since April last year, said: “We had to work very hard, it’s not easy to come here.
“The passion they put in for every ball made it tricky but we took it very seriously.
“Alexis Sanchez can play everywhere, he’s fantastic. The attitude he shows, he fights for every ball.
“He’s a star and it’s amazing to have him with us. This is a team and today we did it well.”