Definitely can. But there are other HCs out there who still led their teams to successful season when the starting (good) QB went down. Caldwell is not part of that group. I’d be more interested in Caldwell as the OC, not HC.
How about Caldwell as HC and some up and coming offensive guru as the OC?
To me, that is the best possible solution. Caldwell gives this brainiac his first opportunity to have the OC title while still being able to lead him because of his (Caldwell's) offensive background.
A defensive HC would be unable to guide a young brainiac because he doesn't have the offensive background.
I would be more than ok with Caldwell as HC, a Fangio as DC (he controls the defensive side of the ball) and some young wonderchild who can grow with Fields with gentle guidance from Caldwell.
If Caldwell gets sick, Fangio can be his backup.
Once Caldwell decides to retire, that wonderchild can then become the head coach, having learned the OC position and gotten some insights into being a HC from Caldwell and Fangio. I believe that Fangio will be fine being a DC for the rest of his career.