I think this is a big piece, but there's more to it. Chryst's first couple years were largely successful and that was with Stave, Houston, and Hornibrook turning the ball over a lot. Outside of 2019 where Jack Coan went 18/5 TD/INT, their QBs have been pretty turnover prone but they've overcome it.
Chryst was considered pretty cutting edge with the run game, and in their losses this year the run game has been shut down outside of a late, meaningless 75 yard TD against OSU. Mertz certainly hasn't elevated them as expected, but having a good 1-2 punch in Allen/Mellusi and still not being able to run the ball effectively is pretty alarming. They wouldn't need Mertz to be the high 4-star they expected if the rest of the offense could do anything.