Van Gaal handed the skipper's armband over to the England international during the summer following Nemanja Vidic's move to Inter Milan.
"There are always players you put in a team," Van Gaal said in this afternoon's press conference. "My captain shall always play and normally the goalkeeper, so it's not a surprise. Maybe it's a surprise Blackett has played all the games, but he has played well and Mata is a very good player.
"I don't think any player is fixed. Only the captain has more privileges, but no other player has privileges, I think. Every player knows that because you can see that by how I manage my teams."
United will take on Leicester City at the King Power Stadium tomorrow.