Hernandez cost United barely two percent of the £350million spent to rebuild Lansdowne Road but the Mexican looks set to be regarded in as much affection as Ireland's impressive new home after another pre-season goal on Wednesday night.
In front of 49,861, Hernandez scored with his first touch to launch a second-half rout of the best Ireland's domestic campaign can muster, his third in as many games - one of which was against his new club - that offer huge promise for the 22-year-old nicknamed "Little Pea".
Michael Owen also found the net on his first appearance of pre-season but there was nothing for Wayne Rooney during a typically industrious 45-minute display.
Aside from the minor inconvenience of Rooney's blank, the major piece of bad news for United on a night when they were hardly asked to do any defending was the loss of Michael Carrick, who limped off midway through the opening period of his first game since a World Cup campaign he watched exclusively from England's substitutes' bench.
Funnily enough, on the day the anti-Glazer Manchester United Supporters Trust announced the launch of a widespread poster campaign ahead of the new season, there was little Green and Gold in evidence at a ground where green is definitely the colour.
There was plenty of red though as the Irish supporters showed which English team remain the club of choice on the Emerald Isle, and plenty of sympathy for Rooney too as he kept himself busy on his return to action after that sorry World Cup and those fairly damning pictures.
Rooney should have found the net before the break. Instead, he dragged one effort wide from an acute angle before blazing a better chance over when he latched onto the rebound from Darron Gibson's shot.
A goal would have done wonders for Rooney's confidence after such a depressing summer. Now he will have to wait for Sunday's Community Shield encounter with Premier League champions Chelsea at Wembley.
For United, it hardly mattered.
The standard of their opponents could be measured by six representatives from a Bohemians side surprisingly beaten by Welsh outfit Total Network Solutions in the second qualifying round of the Champions League in Damien Richardson's starting line-up.
United's opener arrived in suitably farcical manner when Gavin Peers cut out a Park Ji-sung cross bound for Rooney at the far post then got himself in a complete mess as he decided what to do next.
By the time Peers made his mind up, Park had closed in. The South Korean stuck out a leg to block the attempted clearance and the rebound looped in.
There was a touch of class about Owen's effort though as he tried to nick the ball away from Ken Oman inside the hosts' box, then calmly lobbed into the net when he was presented with a second chance thanks to a kind flick off the defender.
The interval exit of Rooney and Owen offered a chance to Dimitar Berbatov, the record signing with so much to prove, and Hernandez.
A total unknown outside his homeland when he signed for United at the back end of last season, Hernandez gave fleeting glimpses of his talent during Mexico's run to the last 16 of the World Cup before scoring twice in as many appearances on the Red Devils' recent North America tour, including one against his new club for Chivas as he made his farewell appearance.
It took him less than 100 seconds to extend his 100% record as he latched onto Antonio Valencia's low cross and drilled home his side's third, the reception both before and after highlighting the impact Hernandez has already had for the Red Devils.
Resistance was ended by Valencia just before the hour, with Park, Jonny Evans and a Nani penalty adding further efforts, although St Patricks' Dave Mulcahy probably enjoyed the greatest moment of his career when he drilled home the hosts' consolation.