Not that easy. All have between game activities they go through, including when they thrown in between starts. Maybe they throw two days before a start (I don’t know, just tossing a hypothetical out). Houck would have thrown yesterday in prep for tomorrow and Thursday’s starter throws today. Using Houck in relief tonight messes that up completely. Even if it’s 3 days before a start, you are still upsetting that schedule.
Houck is prepped for tomorrow, no way you use him tonight.