I usually assume that most of the formulas that filter into their main algorithm is the same for SLB and HBD, since it wouldn't make sense to be different in most areas. So with a grain of salt I'll mention that in SLB a player's offensive and pitching performance is handled on a curve not a linear line. So a .300 hitter at 99% would probably hit .2994 or something like that rather than .297. As a player fatigues more the curve picks up pace. As far a defense goes, the penalty is severe much more quickly. Its probably linear in some way rather than curved, so as a general rule, I don't mind playing slightly fatigued guys at lesser defensive positions, but hesitate to play them at premium ones like SS.
Again, thats just how it works in SLB and I don't have concrete proof that thats how it works in HBD, but its just one of those things that works just fine in the one game and it would be odd of them to write completely different formulas just for the sake of being different for this game.