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."
@carrick4england , the issue is where do you play him? RvP and Rooney are both better players, it's as simple as that. You cannot play all three because it simply wouldn't work. Fact is, Hernandez game is based on percentages. He's plays right on the shoulder of the last man but he doesn't bring anything else to the table at all. His touch is iffy, his passing is woeful, his hold up play is worse than terrible. Yes, he'll finish the chances you create, but modern football requires its strikers to contribute more than just goals. Welbeck at least brings some versatility with him. Hernandez does not. Great little player, but if he needs playing time, he needs to look elsewhere.- HarryBoulton