He knows his arm well. He pitches on the side every week between starts. Pitching on one day less rest happens often for pitchers in the playoffs so it’s not a super rare thing.
A must win game 4 is more valuable since there is no game 5 without it.
My point is it would be a shame to lose in 4 games and only pitch Skubal once because you were saving him for a game that never happened.