The Spaniard joined the club from Athletic Bilbao for a reported £29 million fee in the summer, but has made just 12 Premier League appearances this season – missing games against Chelsea, Manchester City and Arsenal.
And Van Gaal has now urged the 25-year-old to prove he deserves a spot in the starting XI ahead of regulars Wayne Rooney, Juan Mata and Angel Di Maria.
“I have to compare him with people like Rooney and Mata, for example, so that’s difficult, they are all at a high level - he has to improve,” Van Gaal told reporters.
“But he’s a great guy, he’s a very great professional also, so that’s not the problem - his problem is he has to compete with high level players and Rooney is also the captain so he has a privilege.
“So then the other places are the place of the position in this system that we are playing now with a number 10 and that’s Mata and that’s Di Maria. That’s difficult to compete against but he did already know that at the moment that he signed for Manchester United.
“When you sign for a top club like he did he knows that he is had to fight but he is fighting.
“He is coming back, there were a lot of weeks he was down but now he is coming back.”
Herrera last started a Premier League game in September – during the 5-3 loss to Leicester – and has been restricted to substitute appearances since then.