I think this can be fixed with a different coaching strategy, you don't need to buy players off the transfer wire.
If you have 3 players and place them in all 3 spots in your lineup as starter-sub-reserve for the same position, that player should get all 48 minutes at the position you wanted as long as your choose 'strictly follow depth chart'.
I've done this several times for training and it's worked out every time. If you put the coach in charge with a blank lineup then literally anything can happen.