ander herrera put manchester united ahead just before half-time with a low left-footed effort .
wayne rooney doubled united 's lead with a beautiful half-volley on 79 minutes at old trafford .
christian benteke pulled one back for the visitors a minute later after a rare david de gea error .
herrera added a third in the closing stages of the first half to complete the scoreline for the red devils .
player ratings - herrera shines as louis van gaal 's side triumph to move above manchester city into third .