AD getting hurt would be much worse than Whit getting hurt. We don't have an adequate replacement for Donald. Donald is the engine of the D. Their whole QB pressure system is predicated on him. And more importantly there is no one behind him that can remotely do what he does. Brockers isn't strong at getting pressure. Having him slide over isn't close to a satisfactory solution.
The negative effects of losing a QB is self-explanatory. Even you didn't have a counter for that.
OTOH, if they were to lose Whit, it would be a blow but Noteboom does stand a chance of doing a decent job. You may not like him as a starter but this is the player that both Snead and McVay stated last year was Whit's heir appointment. Clearly, the Rams felt he had what it took to be a starter at LT. Their confidence in his ability to start suggests that he'd at the very least be an adequate backup.
Besides, not many teams have starting LTs on the bench so it's not like the Rams would be worse off than any other team whose LT went down.