Hernandez has been a bit-part member of David Moyes' first team this season - making just four Premier League appearances - but has scored three goals in two League Cup outings.
And Real Sociedad's Vela, who made the move to Spain after failing to establish himself at Arsenal, believes his compatriot may need to follow a similar route.
"Javier is my good friend - and I hope he gets lots of chances this season at Manchester United," he told The People.
"He is one of the best finishers in England and deserves chances under the new coach.
"If he plays regularly he will score, so I don't totally understand why a player of his quality has not played more games.
"If he doesn't play regularly he will need to think about leaving.
"You always want to succeed at the biggest clubs - but if he is not given the chance there will be big teams in England and Europe wanting him."
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