Herrera struck with just under half an hour remaining to complete the turnaround after Alexis Sanchez had cancelled out Dele Alli's opener.
And after going two seasons without a major trophy following Alex Ferguson's departure in 2013, United remain on course for their fourth honour in three years.
"I am really happy, of course, for the goal, for the victory," Herrera told the BBC.
"We are in another final. We are getting used to being in the final, which is very good because this club is about titles and playing finals. We are very happy.
"[This was] against Tottenham and, until the last minute, the last second, the centre-backs did an amazing job. We are very happy."
United will be confident of getting another result at Wembley against either Chelsea or Southampton in next month's showpiece, with defender Chris Smalling pointing out that his side continue to excel in big matches.
"[Mourinho told United] to play our game plan, to know what we came here to do," he added. "We turn up in big games and we've shown that we turn up in these big games recently."