So, your question really is: does the concept of in-game pitching fatigue exist in HBD?
You'll get different answers from different people.
One answer that I remember that came from WIS at some point in the past, either in a dev chat, or in response to a ticket (I forget which) is that the pitcher's effectiveness is based upon their fatigue at the start of the game. So if he starts at 100%, he pitches at 100%. Which, if true, means that there is no in-game fatigue.
Others may disagree.
I have no definitive data or opinion either way.