United bounced back from going behind to Dele Alli's early goal at Wembley, as Alexis Sanchez and Herrera returned to hand Jose Mourinho another chance of claiming silverware in what has been a barren season for the Portuguese coach so far.
"I'm really happy of course. We are in another final and we're getting used to being in finals," Herrera told BBC Sport after the game.
"This club is all about titles and finals. The centre-backs did an amazing job."
United will be looking to claim a 13th FA Cup crown and will now face either Chelsea or Southampton in the final.