How much ?

It doesn't seem to be too much, as many teams still win when playing players out of position.

In fact, when deciding where to put everyone, I didn't even look at their secondary positions, but I'm 2-0. So while there's a chance they won't play as well, I've seen nothing to make me believe that it's major, at least at this stage of the game.

those are game ratings and not positional ratings. They reflect the players actual performance better and they decrease due to the loss of stamina over a game.