I've been watching baseball all my life, and if there is one consistent myth about pitching, it's that you can take a guy who runs out of gas in 5 innings and leave him out there to be hammered in the 6th and somehow if you do that repeatedly he will magically develop 'endurance'. Your endurance doesn't come from pitching, it come from all the conditioning done the rest of the time, the act of pitching at the MLB level is demonstrably destructive to the health of the human body, not productive. If the player's overall program isn't cutting it, repeatedly pitching him to exhaustion once a week isn't going to help, despite the habit that every manager in the the majors seems to fall into.
If you have 5 inning pitcher, you have a 5 inning pitcher. You aren't going to do anything about it mid-season.