Hernandez stepped off the bench to slam home with five minutes left and earn United an invaluable 1-0 victory.
“The way he took his chance, it was like shelling peas. It was so natural to him,” said a delighted Ferguson.
“He has work to do on his physique but you cannot take away from the fact that he is a fantastic finisher.”
Hernandez was thrilled to score his first competitive goal for the club and said he used to watch the Champions League in his homeland Mexico before signing for United from Chivas for £7m in the summer.
“It was a great night,” added the 22-year-old. “I am so happy about the goal but also it was important to get the win and the three points.
“I always used to watch the Champions League when I was back home in Mexico. It is one of the most important tournaments in the world, so I was always keeping up with it.
“The goal will give me so much confidence. I am only at the start of my career with United and to score so early in the Champions League is a big boost.
“It is great to be a Manchester United player and I want that to continue for a long time.”
Ferguson also praised the performance of Rio Ferdinand at the Mestalla, the returning England captain proving a calming influence on a previously leaky defence.
“You’ve got to give us credit for the way we defended,” added Ferguson. “They’re not top of the Spanish league for nothing.
“Rio needed a game like that. He has the experience, which was vital and the ability to keep the ball, which is important. His experience helped us. There is no question about that.
“For his first real game in a long time we are very pleased with that. He has not played for five months. He had games against Rangers and Scunthorpe, but this was a real game.”opean campaign is back on track after their disappointing 0-0 draw at home against Rangers in Group C two weeks ago as they now sit joint top of the group with their next two fixtures against Turkish side Bursaspor.
“It’s a great result for us. It’s not easy coming here,” he stated. “Rangers also won tonight and have got Valencia in a double header next, so it’s important we kick on now.”
United had only beaten Spanish opposition once in 18 previous visits to the country. –– BBC Sport.