Argentina's Juan Martin Martinez has failed to recover from injury in time for the Rugby World Cup in September.
Pumas coach Santiago Phelan has named his 30-man squad for the World Cup in New Zealand, confirming Hernandez's fears when he wasn't included.
The utility back injured his right knee while playing for French side Racing Metro against Bourgoin in March.
The 28-year-old underwent surgery and rehabilitation in a bid to be fit for the World Cup but he has officially been ruled out.
Nine other players, including Agustin Figuerola, Genaro Fessia, Lucas Borges, Martin Bustos Moyano, Eusebio Guinazu and Rafael Carballo, were also left out of the Argentinian squad.
Argentina continue their World Cup preparations with a warm-up Test against Wales at the Millennium Stadium on Saturday, August 20.
