The 25-year-old has featured 16 times in the Premier League so far this season but he has mainly been used off the bench, making just four starts.
Hernandez has been linked with an exit in recent months, although his agent claimed that the striker had turned down offers from a number of European sides during the January transfer window.
The Mexico star has two years remaining on his current deal with United but Herrera says Hernandez is unhappy after speaking with him on a recent tour of Europe.
"He seemed well but he is a bit frustrated because he doesn't play despite training well and the truth is that when he does play, he makes a huge effort on the pitch," he told Raza Deportiva.
"He is a bit frustrated because the current manager doesn't give him many chances, but he is committed and doing what he can to become a starter.
"He likes to work and likes to show determination to achieve his goal."