Two days is arbitrary, but if you're not allowed to add a guy before he makes his debut, there's going to have to be some arbitrary point in which you can add him anyway then right? Otherwise when would it be the right time to add a guy who is debuting? Immediately as the game starts, after his first AB is completed in real-time, right after the game ends? The arbitrary point is exactly two days after their debut at 8 pm ET so that it's consistent for all owners and gives a fair chance for everyone to realize the player is there if the commissioners missed adding the player to waivers. You could argue one day would be enough I guess, but it wouldn't really change anything as it would still be at some arbitrary time to be fair.