The 24-year-old scored the winner as United recorded all three points, but the Mexican was in an offside position when he converted a Rafael cross from close range.
"This game could have changed the title. That's why it was important we didn't allow them to get ahead," he told the Evening Standard.
"It was vital to get the three points at a very tough place. We are in there in the fight to win [the Premier League]."
Hernandez has scored three goals in his last two games for United.