Van Gaal didnt like him. Herrera was not robotic and safe enough for him, he was too instinctive. He had to change his game a lot to fit his philosophy, and sometimes he didnt play to van Gaal's liking and got dropped. He could score a goal and make an assist, but with van Gaal other things matter more, like decision making and positioning for example or if you exactly played like he said to you. So if he would be box to box and played terrific, that could mean the bench for him for a few weeks, because van Gaal wanted something else. I remember when he always told him to take a touch before a shot or pass instead of going for it directly.
Mourinho was more baffling. First season starter and played really well, then bought Matic and Jose preferred Pogba, Matic and Fellaini to him. It is quite a weird one, since he is a Jose type of player with his character and attitude, maybe not play style with his lack of pressing. Then the talk about mad dogs when he benched the mad dog in our squad. With Jose i dont know if he is genuinely stuck in the past or if he is doing things wrong on purpose. His interviews were garbage here, but then you watch him as an expert, and he knows everything and talks well. As soon as he went back here as manager, he did baffling things. Jose also never really had a starting eleven here, chop and change on a weekly basis. I dont think he knows what he is doing after a while in a job nowadays and just makes the wrong decisions again and again.