United trailed 2-0 inside 20 minutes but Hernandez pulled a goal back five minutes later and grabbed a second-half winner after defender Jonny Evans had equalised.
Asked for his thoughts after another dismal start - the eighth time this season United have fallen behind, Hernandez joked: "I was thinking 'oh, we just need to score three more goals and then we get the three points'!
"We started a little bit badly but thankfully we scored three and we will look forward to the next game."
He added on ITV1: "I've learnt a lot here and when I get the chance I need to do my best."
Hernandez had another goal disallowed for an offside against Shinji Kagawa in the build-up.
And he said: "It wasn't offside but it's not important, all that matters is the three points."
Despite United having to again come from behind, manager Sir Alex Ferguson defended his decision to field a lightweight-looking defence including England midfielder Michael Carrick.
Rio Ferdinand was left on the bench but Ferguson told ITV1: "I think we did the right thing. It wasn't Michael Carrick's fault, the first goal, he did his job well.
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"I think you've got to look at the big picture - Rio, his age, with a big game against Chelsea coming up, Chelsea (in the Carling Cup) on Wednesday then Arsenal.
"We've got Vida (Nemanja Vidic) out, (Phil) Jones and (Chris) Smalling not available - Carrick's not the best but he does his job well."
Ferguson added: "It's been the story of our season at home, starting badly and conceding goals, but we've rescued games.
"It's the front players who are doing that for us, some of our football tonight [Tuesday] was terrific, but it is a worry that we're losing those goals."
He admitted Hernandez's headed double had given him a tough decision to make on the nature of his frontline in the future.
"He trains magnificently, he's improving all the time and he's got me thinking," he said.
"With him, (Robin) Van Persie, (Wayne) Rooney, (Danny) Welbeck... I don't know what you do with them, to be honest."