Herrera was dropped because Van Gaal didn't think he could do the job as an 8 and felt he wasn't good enough to displace Rooney or Mata out of the 10 role.
“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,” said the Reds boss.
“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.
http://www.manchestereveningnews.co...ander-herrera-must-improve-manchester-8597120
As for the main point, we played the same system before, with Mata (10) and Fellaini (8) or Rooney (8) and Fellaini (10) and we were fine. This is when we played Chelsea and City, respectively. Therefore, the opportunity was there for him to use Herrera and he didn't.