Having been felled by Joe Gomez in the box, Herrera stepped up to drill the resulting spot-kick home 20 minutes from time, as United went 2-0 up before Anthony Martial and Christian Benteke both netted superb goals.
Herrera deputised on penalty duties in the absence of the injured Wayne Rooney and put in plenty of practise before being called upon from 12 yards in Saturday's Premier League clash.
Asked by MUTV if had taken a penalty before, Herrera said: "Never, never. But the manager [Louis van Gaal] last season gave me the confidence.
"He told me I had the quality to take penalties and I have practiced every week. And, when I received the penalty, I thought the number-one taker isn't on the pitch, I'm the second taker so I'm happy for that. But, next time, if the number-one taker is on the pitch I won't be taking it.
"I tried to hit it as hard as possible and it was a good goal but the most important thing was the win."
The significance of the result was not lost on Herrera, as he added: "It's not only a victory.
"It's a very special day for us and our fans. When I arrived here, I realised what this game means for our fans. I have played three games against them [Liverpool] and beat them in every game, so I am very happy."