Herrera switched to Old Trafford from Athletic Bilbao in a big-money move before the start of this season, but has made just 12 Premier League appearances, half of which have been as a substitute.
His last league start came against Stoke City in early December and Van Gaal was displeased with questions about Herrera's standing at the club.
"Three weeks ago it was [Radamel] Falcao," the Dutchman said. "Next week it will be about another player who is not playing. It is unbelievable.
"I can only line up 11 players.
"It is not so interesting that you talk about the pounds [fee paid].
"Paddy McNair is also important and he cost nothing, so I do not agree.
"I am not discussing this [Herrera's omission] with you."
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