Apparently seemed to be lecturing Herrera after the game on Saturday about his positioning. Didn't see it, but was reported by MEN journalist Samuel Luckhurst.
I haven't read Luckhurst's article, but I suspect positional awareness/discipline is the reason why Herrera hasn't always been first choice. Herrera can drift from his set position, and Van Gaal appears to be fussy about players' positions and them fulfilling certain roles.