Hernandez has been a revelation since moving from full-back at the start of the World Cup, producing two very different displays against France and Namibia.
Whether pumping the ball in the air against Les Bleus or splitting open Namibia's defence, the Stade Francais back has looked a class act.
O'Sullivan, speaking on the eve of Ireland's last-ditch attempt to reach the quarter-finals, believes Hernandez's form has been key to putting Argentina on the bring of the knockout stages.
"Hernandez has had a huge influence on the success of Argentina in the World Cup as a fly-half," he said.
"I know he's always aspired to playing there even though he's played a lot of football at full-back.
"But he's an excellent footballer, a player that can make the transition from 10 to 15 without any hiccups, as we have seen.
"He runs the game well, he's a threat on the gain-line, he distributes well and he's got a huge boot, so he pretty much ticks all the boxes for a fly-half.
"His change of positions has made Argentina a stronger team."
Installing Hernandez at fly-half has provided a sprinkling of gold dust to a backline that is already bristling with attacking options.
Felipe Contepomi, the previous incumbent of the number 10 jersey, has been shifted to inside centre where his playmaking skills are still able to flourish.
Ignacio Corleto is a world-class strike runner at full-back while Lucas Borges and Horacio Agulla provide genuine pace on the wings.
But for all the threat Argentina possess behind the scrum, it is their forwards that will instil the greatest fear into Ireland's underpowered pack.
Using the pick-and-go tactics that have served them so well, the Pumas forwards will pummel away at the heart of the Irish defence - accumulating valuable yards in the process.
O'Sullivan believes discipline will be crucial at the Parc des Princes tomorrow and knows Ireland must be ready to confront the physical challenge head on.
"The key is that the first collision is the most important one in terms disrupting their momentum and their rhythm," he said.
"There will be times in the game where they'll have possession and they'll hold on to it. They're very good at that.
"We're playing a side that's ranked up in the world with France and ourselves so we're under no illusions that they're good at what they do.
"We must keep our discipline and not give away silly penalties. It's about getting a balance to what you're doing.
"There will be times in the game where we won't have the ball and we'll want to have it. We've got to be sensible and wait to get it back.
"When we don't have it we've got to get about the business of getting it back without giving up scores."
Ireland need to win by more than seven points while scoring a minimum of four tries if they are to reach the quarter-finals, but skipper Brian O'Driscoll is refusing to consider the permutations
"I don't think we can concern ourselves with scoring four tries," he said.
"First and foremost you have to think about winning the game. If you start thinking about racking up points against the likes of Argentina then you're in trouble.
"If things start to open up then fantastic, but invariably games against Argentina are quite tight.
"Putting up a record score against them in a tight, pressurised situation like this is unlikely."