Hernandez - nicknamed Little Pea - scored 13 goals for United last year as he forged a formidable partnership with Rooney.
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The Mexican scored many match-winning goals which consequently helped the Red Devils achieve their illustrious 19th title.
"I hope we can improve. Javier's done really well," he said in The Sun. "He's played all summer so he needs a bit of a rest.
"It will take some time before he gets back into it but the really good thing is that we have such great competition for the two places up front, or sometimes only one place. We all have to make sure we work really hard."
The 25-year-old also spoke about the contribution made by Bulgarian striker Dimitar Berbatov, despite the languid striker being consigned to the bench for the latter part of the season.
Asked if he felt sorry for the Bulgarian, Rooney said: "I want to play the games, it's competition and, at the end of the day, it's down to the manager. Berba still has a massive role for us.
"He was our top scorer last season with 22 goals and scored some important goals."
