Hernandez wriggled away from Jose Enrique to touch home Wayne Rooney's corner.
The Mexican's first goal of the campaign came so early in the second-half, United manager David Moyes had barely got beyond the penalty area in his walk to the home dug-out.
It meant it was easy to spot the Scot's celebration, at a strike that should at least quell some of the angst that built up following that awful hammering at Manchester City four days ago.
United's performance was not perfect.
Indeed, on his return following a 10-match suspension, Luis Suarez came close to levelling when his vicious free-kick struck the bar with David De Gea beaten.
De Gea had only just repelled Victor Moses' powerful diving header.
But the hosts had chances to sew up the contest too, with Simon Mignolet producing a couple of excellent saves to deny Rooney.
And after bravely riding the hefty punches landed on him at the Etihad Stadium, Moyes was probably not that bothered about the actual scoreline anyway, just the much-needed victory against one of England's biggest clubs, and United's greatest rivals.
It was hardly a surprise Suarez should start.