I assume nobody on the field knows the exact rule but that's how most umpires would handle the situation. Still seem inexcusable to me to make that pitching change and not see that he's not on your own lineup card in your hand and pinned in your dugout.
It literally feels like this is peewee league where everyone needs to play and get that participation trophy. A 60 game season should not be this hard to manage.
Having never looked at Hech since he was traded, I am pretty shocked that he has been playing in the majors for the last 9 years mostly as a regular and has a career OBP of .290.