Well it's silly which is the point, by that logic if Ibrahimovic is injured or suspended we then have to play Fellaini simply for height. Regardless of how well the other midfielders are playing, Fellaini's not even great in the air.
The West Ham game was down to Van Gaal playing a 5'10'' midfielder at centre back against one of the tallest teams in the league instead of a proper defender, not the lack of Fellaini.
You also don't need to have world class defenders like Rio or Vidic to defend a set piece. Heading the ball is one of the fundamental skills a centre back must possess, Smalling, Bailly, Jones and Rojo are all good to decent at it. And any combination of those players should be good enough to defend high balls without disturbing the rest of the team by trying to shoehorn someone like Fellaini in. If they aren't capable of that then we replace them with defenders who can.