Ok there is a workaround but it's hard to explain!
When clicking on a formation and play (say Special teams then Field Goal) you have around a second whilst the players load up. Quickly find the player you want to sub and click A (on Xbox) which moves you to the players list. Providing you do that quickly before the screen has fully loaded you can sub players.
For example I always swap out my QB as holder on field goals and replace him with the Punter and I can still do it if I'm quick.
Awesome way to explain it, I tried, but failed lol